The Encased-In-Carbonite Story
In real life, when things happen to us, we change--at least a little. Sure, maybe we make the same mistakes over and over, but some kind of change happens, even if doesn't show in our behaviour. That should be true of characters--especially the protagonist--in your stories. Having a static main character, who goes through the whole story as if she's encased in carbonite, is a good way to wreck your story.
The story may have a perfectly good plot and interesting setting and characters. Things happen to the characters and they deal with the challenges put in front of them. But if the main character ends the story without having learned something, had an insight, an emotional reaction, grown or evolved in some way, the story will fall flat. Something will feel 'off' and your reader will wonder why they bothered to read it.
The evolution of major characters is called the character arc or growth arc. Every story needs one, even if it's subtle. That doesn't mean that a flawed character has to become perfect, or a supervillain has to change into a superhero. Not at all. In fact it's very interesting to read a story where the main character regresses in some way--maybe ends up more depressed or angry or cynical. It's trickier to pull off because the reader will always root for the character they identify with, but there's nothing wrong with that kind of "negative" growth arc. The character arc can consist of growth, failure, or subtle shifts in worldview, but there needs to be some kind of change.
The good news is that this is an easy fix. After the first draft, look for the growth arc as you're doing your first revision. If it's not there, put it in. Show your reader how the experiences your character went through have affected them. Does it make them want to change their life in some way? We don't necessarily have to know how, but we would like to know that the urge is there. Does it make them surrender to something they've been fighting? Show us, and give us a hint of what that means to them. Does it enliven them, or destroy them? Or perhaps it has done nothing that dramatic. That's fine. Just be clear that it's moved them one direction or another, given them a small insight or a new idea. They should look at their world differently when the story ends.
Remember this, because if it doesn't even have an effect on your character, you can't expect it to have an impact on your reader.