First, your new Corys addition would help old Corys to be more active. And it would gain number in ratio although I don't know how many Platys you have in it. Nontherless, more Corys you have less likely to outnumbered. So they can be more bold and compete better for the food although Corys are known to find the food by smell and not sight. They do find by their wiskers.
Second, more decoration especially more plants would be helpful. Although again I don't know how your tank is planted. The plants would help Corys get their food. First, the plants if planted heavy enough deter the Platys to get to food as easy as open water. Second, natural plants culture some micro organism which the Corys can wisker away collecting small food.
I also think the Corys can really benefit from matured small substrates like sands or small gravels. Because lately I notice that my Corys find food by shifting through the sands. I see them gobble up the sands from their mouth and the grains of the sands come out from the behind the cheek(end of the head before the body part, I don't know the proper term but you know what I'm saying.) So to me, they are almost like filter feeder. They can eat really small food partical well. More than bigger sized food.
So I do feed lots of small food. Something so small that Platy have tough time taking them all. Like micro or walter worms, vinegar eels. If you don't have them, you can try the hard boiled egg yolk which you better watch not to overfeed since they do easily foul the water if it stayed in tank too long. But good filter would take care of that.
I also grind up the food. The flakes, pellets what it may so Platys can't get them all.
If you still have difficulty with the Platy. Take some tank water in the cup and soak the food till dissolve in the water and use turkey buster and suck it up and release it in the lower part of the tank.
And if you think after all this, your Corys have not getting their share of food. Try to feed more often and change the water more often. And always feed after lights out in the dark would help. Since Corys do not need to see to find the food.
If all fail, reduce the number of Platys or rehome them somewhere. And maybe find some better tankmates. Since I am all out of my wits.
I hope something I suggest would work for you.
