I have 1 male and 3 females now they keep breeding, so far I have 3 sets of fry.
Although my male didn't really try to breed when I first got him, I added the 2 other females (him and my other female came together) and he started to try and breed with them, then I got a 55gal and moved all of the fish in the 10 including those platys and fry started popping out!
It could be cause your male's not interested, or there isn't enough space or maybe he's/she's still young.
Get places for them to hide if you want them to breed, also get like hornwort of cabomba for the fry to hide in.