No, they dont have all their fry at once, some of them hold on because some of their fry has developed qucikly more than the other, so the mother lets go of the others, and if you are saying this about the one in the breeding trap, she maybe stressed, because those things are small and dont have alot of room, and she when they are stressed can abort their birth, that is why breeding aint to easy, because you have to hook out the mother JUST before they give birth, and that is hard to predict... Congrat on your new fry, i had a same sort of situation as well happen to me just a few months ago, i was at college, i had just got home and saw two small things swimming in the bottom of my tank, when i looked closer, i literally JUMPED FOR JOY... i had to tell everyone in the house, there was only two though
clearly they were bron sometime in the morning just after i went to college, and the female was in the main tank, this was when my CAE was in the same tank, and he must of got some of the fry, but they have grown up and both swimming around very happly with mummy now, who is due to drop my third batch of fry
But congrats to you mate, trust me, fry are enjoyable and give you lots of satisfaction to see them grow up