You should only be able to apply one DCA of any type to a pet before pregnancy. There is a 100% DCA item in the game, but obtaining it isn't common. The most you can do most of the time is 50%- and it isn't a gaurentee. I'm not completely sure if the effect applies toward twins when using fish eggs or not, or only one of the twins- but I like to try to stack DCA with fish eggs! :)
One strategy to speed up getting deviants is if you did not purchase the female (you bred her yourself) try breeding the female's parents again to get extra females to breed to the deviant making male you're trying with, so it's more exciting. In 12 (no DCA) babies, I usually get 1 deviant per 12 attempts. Sometimes I don't, sometimes I get 2! It works, patience is just required.
If you have doubts about the parents being able to make a deviant in the first place, please post links so someone can help you.