what food (brands) have you tried?
i find that they are not picky about commercially available brands of food. "aquarian" brand is a good one; they also enjoy something different occasionally so i also use "tetra" brand color-enhancing flakes. "wardley's" is probably the least preferred. guppies also enjoy picking at algae on the plants and gravel. they love frozen blood worms, too.
although i have never done this because my tank is heavily planted, my local pet store owner always puts a bit of aquarium salt in his guppy tanks.

thanks Jeff, I have tried several some of which you said .. I guess mine are just picky...lol