Sorry about your problems alison... it sounds to me like the platies have some sort of internal bacteria. it can eat away at them from the inside.
I think, as you are still having problems it's time for a dose of meds. I would recommend Octozin. It comes in tablet form & you work out the water volume & dose accordingly.
The way i did it was to mix some tank water in a jug & add the tabs, let them dissolve, stir them up a bit. Tip it into the tank, save a little bit of the medicated water & put the food in for the day then let it soak for 5-10 mins then tip that in. That way, the fish eat medicated food & it gets into their system quickly.
I'm not saying it will definitely save the rest but you have nothing to lose at this stage. They nay be 'too far gone' if you know what I mean.
I have had a lot of success in treating this way
I hope that helps, keep us posted on how you get on