#1) are you sure you have a female?
#2) Are you sure she is not too old/young to breed?
#3) Try feeding a heartier diet with more protein to help her develop the eggs. Black worms or Bloodworms are a good part of that kind of diet.
#4) What is the temperature of the water?
#5) if all of the above check out okay, try doing a partial water change with adding some slightly cooler water back to the tank. This drop usually sparks Cichlids to spawn.
Hope this helps.