I agree that finding food is their problem. For future breeding, try using smaller tanks - until you figure out how to evenly distribute food.
Another factor might be general health. Weaker fry will not move much and will likely starve, regardless how you keep water quality or food.
Things work differently for each and every breeder. You will have to find what works best for you. In my case, I can't use new clean tap water. I have to age my water for about 4 weeks and let micro critters grow (my tanks are outside). So try different methods, change a few details to your convenience until you find what best works for you.