Character Development

Am I the only person who has issues with writing character development?  I can write and write, and even write about one character (character a) and can be completely happy with the way the character is; I am ready to move onto writing the back story of another character (character b).  Then I get five lines in with character b, and then character a jumps back into my mind.  In a sense yelling at me to go back to him/her and write even more details about the character, or that they aren't happy with part of the story.  Does anyone agree?  Disagree?  Please share with me on how you handle this situation.