Bringing believable characters to life on the page is incredibly difficult. It is one thing to know them in your head and in your early drafts, but it is another to be able to carry them through to the manuscript you submit to agents. Often characters are not fully developed and throw off the balance in the manuscript (i.e. setting is a more believable character than your protagonist), which often leads to them not growing or developing throughout the story.
Most common character weaknesses stem from thin characterization. This leads to: predictability, if we are only given a handful of traits then we’ll know how the character will react in each situation which sucks energy out of the pace and suspense; lack of character growth, if we can’t see their strengths and weaknesses how will the character progress through the events of the novel; likability issues, the reader needs to want to spend time with the main character therefore without depth we hardly feel attached to the outcome; low stakes, if the drive of the character isn’t effectively articulated the stakes (what will the character lose if they don’t achieve what they set out to do?) won’t be realized and the plot will never catch steam.
A great tip from Agent Carole Blake’s From Pitch to Publication:
Rewrite your character biographies using only what you have given them on the page, taking out what isn’t in the novel in its current draft. This will show thin characterization and give clues to what information you need to include about the characters that you know but the reader doesn’t.