How long have you had the frontosa? It may still be adjusting to its new surroundings. What was it fed at the place you acquired it from? (hopefully not feeders). If some type of flake or particular pellet then try feeding him the same stuff and gradually wean him off of it to the food of your choice.
You could try some mysis shrimp (assuming it's not kept with mbunas). Or attempt to feed it (and as a direct result, the tank mates) the same food it is currently rejecting every other day (example attempt to feed on monday, if it rejects it, fast the tank on tuesday and try again on wednesday, etc). Hunger will motivate it to accept the food in time. It's best to feed them food that will sink to their level than any floating stuff (some may be reluctant to venture towards the surface and by outcompeted by tank mates, while other fronts aren't afraid of venturing toward the top but may swallow air at the surface and contract 'the float').
When fully adjusted to the tank, frontosa's pretty much eat anything; I haven't found them to be too fussy. Mine eat NLS pellets, frozen mysis, krill, chopped raw shrimp, flakes, etc (not to mention fry born in the tank).