Need more info. Are you keeping the fry with the other fishes or keeping them seperately?
I have currently 6 Sunset Mickey-mouse Platy fry. 2 of them are 12 days old where as 4 are just 2 days old. The 12 days old fries have really grown up. I put these 2 seperately within a breeding net and 4 other 2 days in a seperate net. They all are doing good. I am putting Brine shrimp and Flake dust as their food.
So if you are not keeping them seperately, please do so.
If you have already kept them seperately then most probably they have bacterial / fungus problem which are very common. Then you can try with adding some aquarium salt (1 teaspoon per gallon) and Bacteria / Fungus clear all purpose medicine from any fish store. Put 1 tablet / 10 gallon for continuous 5 days. I think that will help you.
Let us know how it goes.
Good Luck with your fries!