Remember when you were a kid playing superheroes and someone in the group always decided that their superpower was to have as many powers as they wanted? You'd play one or two games and then someone would whine and the game would end because it is simply unfair for someone to have every superpower. They are unconquerable so we all know what is going to happen--they will win and everyone else will lose.
The same goes for your characters. Its easy to get so attached to your main character (or any character) that you make them as perfect as possible. Maybe they always have some sort of witty retort so they never stumble in their dialogue and it sounds unbelievable. Maybe they are a six-foot, tall, dark, and handsome who can beat up anyone they encounter and never drools a little in his sleep. Maybe they have struggles but the problems are over too quickly because the writer is too afraid to leave them hanging in that uncomfortable situation so the tension comes off as superficial drama instead.
Be sadistic! Make your character endure as much as humanly possible in whatever unfortunate circumstance they are in. Readers want to see them succeed, but it only matters that the character succeeds if the situation is difficult to get out of. To put it bawdily, tease your readers before that oh-so-important climax.
Here is the result of a short exercise I did that somewhat demonstrates this point. It was written for a different exercise (write a scene in which your character is humiliated) but the gist is the same.
For any thirteen-year-old girl, life is awkward. The gender barriers begin to bend and deflect each other in ways she's never known the day that hair begins to sprout beneath the pits, all over the legs and even...down there.
Allison Carson was one of those lucky girls whose parents allowed her the liberty of a shaving razor at twelve, before that embarrassing hair became visible. They bought her supermarket tabloids to spare themselves the gruesome parental duty of educating her in the ways of sex appeal. They bought her clothes from American Eagle and Abercrombie&Fitch until she insisted on shopping by herself at which point they allowed her a hefty sum of spending money.
Because of her headstrong ways, her doughty looks, her spotless fashionable clothes, Anne Carson, twelve-years-old, had many friends. But the day Anne Carson turned thirteen would change her life forever.
She awoke feeling as though she'd face-planted off the cherry-drop bars. Allison was the master of the cherry-drop. Her long, slender legs gave her more heft than most in swinging her body round to land on her feet like her cat Minxie.
But this morning her legs did not feel long and slender. They felt like thirty Minxies had taken cat naps all over them all night. They were heavy. Her throat felt thick too, like a boa constrictor had wrapped itself around it and eaten the thirty Minxies. She groaned when she sat up, finding it difficult to open her eyes as she did every morning. She slammed her hand down on the alarm clock, but what she saw there surprised her so much that her eyes shot open wide.
There, where her doll-like hands with the slim white fingers and polished nails should be was a chubby stub of hand with five fat fingers sticking out, each a little too far apart as if making room for the extra meat in between. The knobby knuckles were now only dimples above the finger joints. The polished nails had almost disappeared beneath the swollen cuticles. The really scary thing though was that when she moved her hand this strange hand moved as if it were hers!
Allison Carson ran to her mirror and screamed when she saw that where her beautiful face and body should be was a fat, thirteen-year-old girl who would never be able to do a cherry-drop in her life.
Poor Allison Carson. What will she do? Where does she go after hitting rock bottom? How does her outlook on life change? Does she give up? Proceed on a different path?
Here we have a character in misery who needs to find out what to do now that her life is no longer perfect. She will have to change herself to adjust to this new body and the undoubtedly new lifestyle that will come from her transformation. This is the situation that gives you a round character, someone who grows from their misfortunes for better or worse. Try it out. Put your character in misery. Imagine a time when you were the angriest you've ever been and take it all out on your poor, sad character. Put your pen on the page or your fingers on the keyboard and type for ten minutes without stopping!