Sounds like fertility problems. You could try pulling the next spawn & hatching them in a seperate container with an airstone, with some anti-fungals added to rule out a fungus problem.
Harder water with a higher pH seems to cause problems witrh fungus. When I pull spawns for hatching I add 7 drops per gallon meth blue, 3 drops per gallon acriflavine+, & 4 drops per gallon Maroxy. This keeps fungus at bay, and keeps any that starts from affecting the other eggs or wigglers. I start 50% water changes on the 4th day after spawning.
If this doesn't help either the male or female are infertile.