Rotate foods, too, so they are getting all sorts of different things. Fresh foods are good, too. Try all sorts of fresh or lightly cooked vegetables. Try also a shrimp (grocery store) or prawn. If you can find mosquito larvae, aphids, earthworms and other bugs, these are usually well accepted by the fish.
However, if you are already feeding quite a lot and the fish are not breeding (and lets face it, Platies and Guppies are the rabbits of the fish world. They will breed under all sorts of conditions) then there may be some other issue going on.
You might start by worming them, in case there are some internal parasites. Are they showing any symptoms that might suggest something like this? Eating and not gaining weight or breeding suggests that something else is benefiting from the food.