Siamese Fighter breeding question

I have successfully bred betta splendens roughly about 15 - 20 times. At the start there were a few problems but nothing major , and I learned from that. I have successfully raised between 30 - 50 young fish to a good size before selling them to the pet shop for a very little profit, but its a hobby and I enjoy it. My question is this: The last time ( about 6 months ago) a newly purchased male and female bred or me successfully.I fed them the usual rich foods like earthworm, bloodworm, tubifex and SOME flake food. Everything was fine for a couple of weeks and then the fry began to dwindle. I was left with 4 fish out of roughly 40 and 3 of them have seemed to have developed swolen bellies and have a swimming defect, 1 is 100% perfect, all are females.I'm simply baffled. The parents were both nice sizes and bred perfectly. Would there be a genetic defect in one of them??

I sorta agree with sabersquirrel except on the issue of freeze-dried foods. They will often cause constipation, thus leading to swim-bladder problems.

I believe, ideally, betta fry should be fed microworms, no? Try cultivating your own, as this is supposed to be the best thing for them.

Was the temperature kept at a constant ~80 degrees to promote the healthy environment they prefer? Did you test the water for ammonia/nitrite/nitrate/pH values? Was everything normal? If your water quality is fine, I'd suspect bacterial issues. Did you happen to quarantine the new mother and father bettas before you bred them?