I suggest adding some shrimp pellets or other meaty food to their diet.
Cories are omnivores, but they need a primarily meaty diet. Their digestive systems don’t properly process plant materials. Though if you want, you can give them algae wafers on ocassion if they like them. It won’t hurt them, but it’s not the best staple food.
I feed mine HikarI Sinking Wafers once a day, along with frozen bloodworms or brine shrimp once a week. They were on API Bottomfeeder Pellets for a long time and it worked well for them, I just wanted to give them something higher quality.
Mine get a variety....as do all of my fish. I only got corys this past week, so your millage may vary.
Mine do eat all the leftover flake that I feed the other fish. I also put freeze dried blood worms, tubifex worms, bug bites in the tank. So the corys get any of it that hits the bottom (and I make sure to feed enough that food reaches the bottom for them). I do have the sinking wafers that are supposedly made just for corys and I have put them in. I put a couple of shrimp pellets in every other day as well.
I really try to keep a wide variety of foods going into the tank so everyone gets what they need.