it helps to have a frozen food, or live food, that they will go after with gusto. then you can add additional bits of other foods.
i made one of the paul B style feeders for the baby brine shrimp. this was useful because it got my mandy used to associating a specific area of the tank with food. after some time, when the pumps were turned off, she would be more inclined to visit this area and wait for food. it wasn't perfect, often she would just decide she wasn't interested at that particular time, but it was useful for introducing her to things.
i also would feed the other fish on the other side of the tank, as they tend to be more aggressive and scare her off. she is mostly interested in live food, but will enthusiastically eat nutramar ova. less interested in frozen cyclop-eeze, but will still pick, and doesn't seem to be very much bothered by other things like mysis.
your mileage may vary. they're all worth a try.