Try feeding shrimp pellets. Cories aren'y strictly scavengers/algae eaters like plecos. It may be that your Cories are getting plenty of food from whatever you are feeding your other fish. By the way, you might want to get at least one more Panda Cory since they feel more secure in small groups.
my corys are soooo fat. I never see them eating when I feed them because theyve been eating all the scraps all day and I suppose they just are'nt bothered because they know that they can eat at any time. put it this way. If they are lively and still healthy they must be eating!