There are two types of main characters:
An ordinary character to whom the extraordinary happens (think Jim Halpert from The Office … and we can debate at a later date if he’s truly the MC), or a character who’s already a hero (think Jack Bauer).
One of the mistakes I see new writers make (and something I also did early on) is making their main characters pretty much perfect. They have a zinger for everything, everyone around them thinks they’re fabulous (except the antagonist, who’s a total loser anyway), and they adapt easily to changes in their life. No one wants to read about perfect characters. Perfect characters are flat and boring. Give your characters flaws. And if you’ve got the “hero” variety of main character, be sure to give them something “human.” A weakness, a bad habit, etc.
Something I’ve recently started doing, which is a technique I learned from the fabulous Susan May Warren and Rachel Hauck, is finding a lie for my character to believe about him or herself. Like in the manuscript I’m working on now, Anna believes there’s nothing special about her. She’ll spend the rest of the book believing this, until toward the end when someone (haven’t quite worked out who yet…) will teach her about what makes her special.
So if your main character is falling a little flat, first make sure you’ve given them some flaws. If you have, try giving them a lie to believe about themselves/the world around them and see what that does for your story.