When I first got my percs it took them about a week before they would eat anything. Then they slowly started accepting food. Now they eat anything i give them.
One thing you can try if available to you is live brine. This seems to stimulate feeding in most fish.
I like to offer different foods on different days. I offer them variuos frozen foods, flakes, and live brine. They eat it all.
They will also nip at the seaweed selects that I clip in there for the tang.
Also a varied diet is more healthy for the fish.