Cories must be fed. They will not find food by scavenging in the substrate (unless they are getting food that you drop in)...a lot of people think that they are scavengers so they will just survive by scavening - and that isn't the case, because as longtime mentioned, there isn't any food that just appears in the substrate...only the food you put in there.
Feed your other fish first to get them full. Throw cory food in at night right before lights out. Whenever you feed yummy snacks like frozen bloodworms, just throw in enough so that you see a fair amount falling to the ground.