How many do you have? What sexes? If you have two males in too close of quarters, they could be stressed by constantly having to defend their territories, and may not want to drop their guard in order to eat.
My m/f combo colored up VERY quickly, within two or three days after being in the tank. I feed them blood worms and
brine shrimp on a near daily basis, occasionally feeding flakes/pellets and smashed peas. They are in a 55gal at 6.7ph, VERY soft water, 82ºF. Also, when I do a
water change, the water I'm adding is usually much cooler(76-78º), but I add it very slowly in order to avoid a shock, and they flare in coloration after that.