I've had this happen, as well as just having a character not respond in the way they might have been intended to do in the beginning.
Try as I might I just can't find the appropriate motivation to get the character moving, and so yes, I develop a sort of writer's block.
I've also had that happen with partners, though for some reason some people are hesitant to admit this. I don't know why, because for me the game/writing ceases to be fun, and I really see no point in it. It's no one's fault. If could just be a difference in writing styles, or there just might not be any chemistry between writers/characters.
Not everyone is going to write in a way to please everyone else, but they shouldn't take this news as personal criticism. It's just one of those things, and I usually try to end the game at that point.