It is possible that you're writing a shorter story (i.e. you need to add more events, a sub-plot, twists, and turns to make it into a full-size novel). Or you may be "telling instead of showing". But things like characterization, foreshadowing, etc will also add size to your novel. Or maybe, you're just a very effective writer?
Count words, not pages
In writing, and especially publishing, it's more common to count words (writing) or even characters, than counting pages.
In order to figure out the word count per page of your novel I suggest you go to a site with word counts for published works or just count the words on a random page to get an idea.
Then you compare word counts of that novel with your text to get a better idea of sizes.
If, however, you still find your novel is much shorter, read on!
Are you an effective writer?
Maybe you are just that effective and you're able to do everything I'm listing below (and all the rest your novel needs) in much fewer words?
Then, by all means, don't add words just to make your story longer. Make it epic by adding more plots and events.
For an example of an extremely tightly written story, check out the TV series, Alias. I'm amazed at what they manage to pack into a single episode.
Or let your text be a short story! That's hardly a failure. Just make more than one and you have a collection of short stories, or maybe they will coalesce into a novel anyway once you've produced a few of them.
Twists and turns
To make your story longer, and usually give your reader a more dramatic read, you can take as a rule to end scenes in bad outcomes and setbacks and only give the character a "win" when it will cause larger risks or leading to another setback, one or a few scenes away.
This creates a meandering story that could potentially take forever to get from "A" to "B", but throwing problems in the way of the characters does make the reading potentially more enjoyable. (Who doesn't want to read about other people's problems?)
Worth noting is that these setbacks can come from both external circumstances as well as internal. What would happen if your main character was a drug abuser, or had phobias, or was a "House M.D." type of character that forced his sidekick (the POV-character? The Watson?) to mend fences all the time, or something else preventing them from just doing the job?
Show, don't tell
As has been mentioned before, showing instead of telling will inevitably use more words.
He was angry.
His skin was taking on a hue of red and his eyes opened wide. He breathed fast through his nose, almost making his nostrils moving and it seemed, if I didn't stop, he'd bare his teeth and lash out or run out.
Scenes and Sequels
To avoid overdoing showing you could use Dwight Swains Scene-Sequel system, following up some or all shown scenes with a sequel.
The sequel is used to "transport" the reader from one scene to the next by telling or at least by employing very minimal word usage. (It shouldn't be more than one or two pages long).
This way you can splurge on the scenes and fast forward between them.
The sequel also adds a reaction to the setback of the previous scene ending. Not all, but some setbacks could be deepened by also showing the subsequent reaction to them.
Add a subplot
Sometimes your text can be really improved by an appropriate subplot.
For an action story, a romantic subplot could give the leads a chance to reveal more of themselves, just as an action/conflict-based subplot in a romance novel can do.
A subplot, of course, will add anything from a fraction of the story to more than half of it (check out "Witness" where most of the movie is spent in the subplot... hmmm or maybe it's a romance movie with an action-based subplot?)
Things to do in a scene
Sometimes the first draft will be short because it lacks things a final draft must have that can potentially be added in editing.
I.e. you may not have a short story once you've edited it to a finished story.
Here are some examples:
Showing character—maybe you have to do more in dialog and action than just getting from "A" to "B" when you have to do it in character Cs typical way of doing things?
"Did you steal the bike?"
"Did you steal the bike?"
"I can tell you for a fact that I did not steal the bike. I wasn't even near the bike when it was stolen!"
"Did you steal the bike?"
"What is a theft after all? Depriving someone of something they sorely need? Was that bike needed? Was it sorely needed? In order to steal one might also suggest that you need to have been there at the movement of the actual theft. Was I there? Am I even aware of the actual moment of the theft? One might suggest in order to steal there must be a need. Did I need a bike? Desperately enough to break the law? I say no to all of the above. There was no need! No desperation! There was no awareness, nor any presence and there was certainly no deprivation!"
Introducing character—falls a bit under the "showing" above but you should take some extra care to polish the reader's first impression of your characters.
Building suspense—using action, description, dialog, character psychology, and pretty much any other element in a story to build an atmosphere of suspense will make the text longer.
And you must have this element in every scene.
The suspense can vary between genres from romantic suspense, to will-they-be-murdered suspense, to comic suspense, etc. Your reader needs to be on the edge of their seat pretty much throughout the novel.
Foreshadowing—falls a bit under the suspense building above. In editing, you know what will happen next, and adding hints and clues early on in the text isn't just great fun, it also has the potential to heighten the quality of the text! So much so that it is a vital component in some types of twists or the reader won't buy it.
And of course, you can't just tell the reader what's going to happen... you need to be subtle and subtility usually uses up more words.
Centering the world and the setting—If you don't create an image of where the people are, and give the place character, you risk having "talking heads"-syndrome. I.e. just dialogue and the reader has no clue where or how this really happens.
You can do this by introducing the setting as we see it the first time and then remind us again with some shorter descriptions on subsequent visits. You can also show the setting by having the characters interact with it throughout the scene.
But then there are more complex elements of the world and setting, such as cultural and political factors that may have an impact on the whole story. (E.g. a catholic priest in North Ireland in the 80ies, or a black person in South Africa in the same decade, etc.)
There could be added complications that your text could or should address, and in doing so, using more words...
Using all five senses—falls a bit under both setting and characterization above. You could describe scenes, characters, and settings via the POV-character using more senses than just vision. How does the antagonist smell? What's the structure of the tabletop in the main character's office? What sounds can be heard? What does kissing the love interest taste like? Or how about fear? What does it taste like?
Working towards a theme and message—Sometimes you would add things in the story that aren't effectively moving the plot from "A" to "B" because you want to work with the theme or the message of the story. For instance, by using anything from symbolism to adding scenes that may not necessarily be needed to defeat the villain to adding a full subplot.
Using symbolism—You can use symbolism to do everything from revealing character to describing setting to conveying theme and message and to do so, you might find yourself spending an extra paragraph on that toaster in the murder victim's apartment because you know, the weather and climate change, right?