Being vague is usually a bad idea, particularly early in the story. In the example, it doesn't make a difference what the author is planning or intends, all that matters is what the reader reads. If all you get is that Holly is avoiding Sam, yes, it does fall flat for the reader. They can't sympathize with Holly, or Sam, so the reading was a waste of time.
In writing you need the reader to identify with the characters in some way, or more generally the reader is ready to like the characters, particularly the Main Character, and you should make them want to follow the MC for the length of the book.
You do this by letting the reader in, on the thoughts and feelings of the MC, and that means you can't keep secrets about why they are doing what they are doing. In the same way you can't be vague about it. (Some authors do get away with it, but this doesn't make it good writing.)
In Third Person Limited (the narrator only knows the thoughts and feelings of the MC) you can have other characters keep secrets that your MC doesn't know, or doesn't even realize exist. Holly's best friend might be hiding something important, and Holly doesn't even realize it.
But don't be vague about the MC. Or, for that matter, about the setting or other descriptions. It is permissible to not describe things that don't matter -- a store can just be a store, a dirt path in the woods doesn't need a lot said about it, even the description of a character may be omitted altogether or require barely a sentence -- but if you choose to describe something because it is important, try to be specific and concrete. a "forty foot pine" is more concrete than "a tall tree".
