Yeah, if the pair wasn't formed from a group and instead, forced to spawn by just placing 2 random convicts together, you will have issues. When I try to breed by being selective (color, size, etc) with my breeders, they always squabble after spawning. Luckily, I have a couple breeders that are "in love" which have great color, size and form!
Angelofdeath, try to remove the male (divider, another tank, etc) for a few days. During the separation, condition the female for breeding by feeding her generously. Then introduce the male and see if they hit it off. If not, remove and try again in a few more days. Or if you have another male, drop both of them in at the same time with the female and 1 will surely win her over. hth