Yes, I have heard that mentioned. I probably shouldn't have made Fredrick so brash, but I've been noticing something.
All of my characters seem to act similarly. Is that the only way I can make a character? Does that make me uncreative?
I don't know if I'm qualified to address your question, considering I have two female characters who are traitorous outcasts, but I hope I've constructed characters with dissimilar enough personalities to offer you a little help.
How did you develop your personality? A lot of it is through your experiences, right? So if you could sit down and write a rational biography of each of your characters, it should theoretically lead to a resultant (and constantly developing) personality. When someone is betrayed, they often become mistrustful and bitter. When someone is successful, they can become prideful.
Look at your characters' backgrounds, fill in the gaps, take into account the fact that the other characters from their pasts have their own personalities, and keep developing personalities based on that. I don't think you're uncreative; after all, your characters have wildly different character backgrounds, and they're all really very interesting. My advice is to let those different backgrounds guide their thoughts, actions, and reactions with a more steadfast hand, rather than how you, the player, might want the characters to think, act, or react.