How old are they? As they get older they start to slow down. Are you letting them have them in the tank or a seperated tank or like a breeding net thing.
I would seperate the female whens she is a couple days away, put her in a other tank, or a breeding net.
If you use a breeding net, the fry can only stay in there for about a week, than they get to big and need more swimming room.
If you really want to breed I would get a 10 gallon and place the females in there and raise the fry in there. What size is your tank now. If you dont want to do any of that than I would get a lot more plants so the fry have more of a chance to survive with out gettinf eaten, because that is most likly why you are only finding one or two at a time, the other fish are eating them!