It can be really tempting when making a character to add complexities to them. Little niches, details, and flourishes that make them stand out from the rest of the crowd around them. As a GM this can be fine, and a good way to show which NPCs are more important than others. After all, a complex character shows a lot of GM time was put into them, and a lot of GM time means that they must be important. Otherwise, why would the GM have put that time into them? However, when you want the character to grow, to develop, and take on a life of their own, those same complexities you put in there to make them stand out can strangle the life out of them.
Why does that happen though? I mean, it stands to reason that the more hooks and possible connections you put into a character, the more room it should have to develop, right? Well, this is true in a sense. As in, if you are writing a book, and have everything planned out, you can custom tailor a character to fit that role and have it develop exactly how you want it to. However, in a game, even as the GM, things very rarely go as expected, and all those carefully laid hooks can suddenly start digging into the character, holding them back, and making them stagnate. You end up in a position where the character can't react because of all the details in their background.
It can be a frustrating ordeal when it happens to you too, if you haven't experienced than just trust me on this. You want the character to go somewhere, only they can't, and even worse, they can't because of all the work that you put into them. The character quickly becomes unfun. As a GM, you'll probably end up dropping them, but as a player it can be harder. Sometimes the GM has been working plans for that character, and probably long time planning type plans - considering how complex the character is - and that makes it hard to drop. You talk to the GM about it, and he/she gets that look in their eyes. You know the one, the "What the fuck, I've done all this work for you and you want to bail?" Now, the good ones will then smile, realize it is a game, and that you need to have fun, but that initial reaction is still there.
So, how do you stop this from happening? Well, if the complications are what are holding the character back, then you need to prune them off. The same design principle you use with making a game system applies to characters. Namely, K.I.S.S., or, in english, Keep it Simple, Stupid! Boil your concept down to its bare bones and figure out what you like. Keep things simple, don't over complicate, and let the character go naturally.
Easy to say, right? So, how do you do it? Well, lets start with the biggest place I've seen most people screw this concept up, the back story. Keep the backstory simple, don't go for a 23 page epic when you can tell the same story in 4 paragraphs with summarization. Now, for some, that is over summarization, but being able to tell a story quickly is an important skill to develop. Also, when you summarize you don't give all the details, not giving details means you are complicating your character, because you aren't tying them down to specific facts. Ultimately, this gives the character room to grow, because as the game goes on you can change those unspecified details in the back story, you can go with the flow of the game and see how the character develops.
The main point here though is, the less you detail, the more your character can grow as the game goes along. You can surprise yourself with new twists, new turns, and new avenues with the character. So, why not give it a shot? Lower the detail level, and let the character grow and shine as the game goes on.