If you have baby guppies in that tank, the platy fry may just be initially hard to tell apart.

Platys andmost livebearers are terrible at reading a calender anyway. If your tank temperature is below 76, they may be a tad slower because biological processes slow down a bit.

A worse case scenario, and probably not that of your platys, would be where livebearers and other fish have seemed full of fry when they were really full of Camallanus. Watch their vents carefully. If there are ever little worms protruding, then they are infested with the highly contagious  Camallanus which don't show themsekves in the fish until 3-4 months after they were originally infected. There are a zillion references to this malady in the Quicklinks. Here's hoping that is not the issue with your fish.