I have... er, had trouble writing everyone except Lois.
I know who Lois is. I have a definite image of her and so I pretty much am comfortable with what I feel she will do and/or say in most situations. Now my vision of Lois is my own and may not jibe with the gentle readers but at least she's not a problem to write.
The main reason I feel I have trouble with everyone else except Lois is because Lois is the only one I actually care about.
Tank (who can't seem to write orginal characters either)