This month, the Skyway Writers talked about the need for an active protagonist. It's a subject vitally important to me because I'm an introvert. If I'm not paying attention, I'll start every story with a passive, reactive character. I still remember the moment when I took a synopsis class years ago and the teacher pointed out my character wasn't DOING anything assertive to reach her goal.
Oh. You mean, they have to pick up the sword and yell charge?
Your protagonist has to drive the story, even if it's a quiet, emotional tale with very little action, and even if they make a wreck of their lives and everyone else's, they have to drive the train. From the onset when they make the decision that sets the story in motion, each decision and action must propel them to the climax and resolution.
That's so much easier said than done for someone like me. I try not to let it interfere with writing a first draft but I pay attention to it in every revision and ask my critique partners to keep an eye out for places where my character's not asserting herself. If you don't have a writing partner, don't despair. You can teach yourself to beware of weak protagonists.
When you revise, take notes in every scene. Is your main character making decisions that drive the plot or is she merely responding to other characters' decisions and actions? Is she following someone else's lead? If so, it's not a bad idea to look at the leader and ask if maybe it's their story you should be writing. Try a chapter in their point of view. Or go back to chapter one, wind your protagonist up, then buckle on that sword and charge!