red oscar not eating

he will eat flake. but i've tried 3 brands of cichlid pellets. he
might mouth them, then spit them out. if i feed flakes the other
oscars won't eat pellets, flakes only. also the flakes don't seem
substantial enough to satisfy a largish oscar. what can i do? is he
in a down spiral? i do regular changes, vacuuming, the water is
clear, and all levels are in optimal tolerances. but this has been
going on about 2 months where flakes or nothing. i have a couple
cory's that i don't want eaten. he will eat feeders, but i've heard
feeders are an empty food for tank kept fish, and should only be
occasional food. i have tried krill, bloodworms when he was smaller.
could he have painful gums or something? thanks