Yep, ground flake worked for me after the first week, liquifry up to then. If you put in both liquifry and a little (I mean real, real tiny) bit of very finely crushed flake, you'll probably see them start to eat the flake when they want it. One thing, keep them in the net for a week or 2, but be careful - fry in a net definately grow slower than fry in the tank, and when you let them out of the net it takes them a little while to figure out they need to hide. I've found after about 3 weeks, the adult platys ignore them and don't think they're food. If you didn't use a breeder (ie: you caught the fry and put them in the net), you've probably missed some fry that are still hiding in the tank. They'll turn up in a week or 2. Don't forget to check your filter, a couple always get sucked in there. When I do water changes, I gently pour a little water through the breeding net if I'm using it, it seems to wash the junk through the bottom of the net. Oh, and put a mini plastic plant in the breeding net - they love hiding in it, and they get easily scared when the adults swim up to the net.
If you're interested, I posted some pics of 2 day, 2 week and 4 week old platy fry here somewhere...