Forewarning, this comes over to me as a lack of planning, so a LOT of my answer delves into how to plan. If planning isn't what you're looking for...skip to the last part, under the line.
When it comes to things like this, I don't like to think in terms of rules. Because of two key reasons.
If you think of them as rules, you'll follow them. And that leads to formulaic writing, which I also think of as bad writing. Even with kids shows that I enjoy, formulas annoy me.
What works for me might not work for you. Add the thousands of YouTube videos you can watch, the books you can read, the lectures you can attend, and the writing courses that are readily available...and you have more conflicting advice than telling all your friends you have relationship problems and asking them what you should do.
So. What I suggest, and whatever anyone else suggests, see if it works for you. No matter how confident they say it, no matter if they are JK Rowling, Sarah Maas, Sydney Sheldon, or George RR Martin. Everything everyone says is advice, and you should treat it as such.
My advice? I'd start with deciding how long this piece has to be. Sometimes this is because of guidelines for a publication, or just an arbitrary number to challenge myself.
Once the length is decided upon, I break the story down into sections, both in my head and on paper (digital, but you get the meaning). The average scene is roughly 1500 words, so you can estimate based on your word count total how many scenes you have to work with (some are shorter, some longer, but eyeballing it gives you an idea what you're working with).
Alright. So you have your total count, your total allotted scene count. Now you break down your story into your acts. How you do that? That depends on the story you want to tell.
There are probably thousands of structures you can follow. They each have unique flaws and strengths, but that's with everything. And don't try to follow them to the letter--again, I hate formulaic writing. If you need a few ideas, try the Hero's Journey, the Heroine's Journey, or just google story structures for a plethora of other examples.
The way I see it, the longer the story, the more complex the plot is allowed to be, with subplots and all that. Short stories don't get that (obviously), so as a rule of thumb, for me, for every 10K words, I allow one subplot. Why is that important?
Because now you have your structure, and you need to start plotting in your tension arc. There are thousands of theories on how to do this. And frankly, I'm not trying to write a dissertation here. So suffice to say, you need to get a feel for writing tension into your story in one sense or another.
So. You have all the above steps done, and you're short on your word count? Here's what I do. I go over each scene, and check the following items off:
Are the senses engaged? Can I see, smell, feel, breathe in this scene?
Is each scene vital to the story? If I take it out, does it impact the story?
Is anything left ambiguous (by accident)? Beta-readers can help figure this part out, or critique partner.
Am I using filler words? Everyone has a different interpretation for what 'filler words' even are. But my rule of thumb is no: feel, see, taste, touch, thought...no anything that makes it feel like the character is telling you what's going on. So no, "John felt her skin, it was soft as silk." And more, "John's hand traced down her naked back. The silky softness of her skin always did things to him, as did her Chanel No 5 perfume." (engage the senses, in other words)
With these things in hand, I go over the story again in the next draft and cut down what needs cutting, bulk up what needs bulking up, and when I'm done I read through the whole thing again.