Every story must have a dynamic character, one who changes because of the action of the plot. This change does not have to be cathartic, meaning that the character does not have to experience a life-altering revelation that affects perception of the world or people around him/her. Instead the change can be subtle.
Perhaps she realizes that her best friend has not been loyal, and so vows to never trust that individual again. Maybe he goes for a walk in the woods and sees the most beautiful meadow in an area that is slated for logging. The character realizes that he/she must act in response to the situation.
Stories also have static characters. These do not change and so are exactly the same as they were at the beginning of the story. Static characters flesh out the details, give the protagonist someone with which to interact, and are sometimes the stimulus to change.
Good stories have both types. Look back at something you have written recently. Does your protagonist experience change? Is he/she different from when the story began? If not, what details do you need to add to give your character that opportunity to grow?
Rewrite those scenarios in which opportunities arose for change, but which were not fully explored. Add in details that enrich the life of the protagonist, rounding him/her out in fuller detail. Make your character dynamic, so that at the end of your story he/she is different from when the story began.