Convict cichlids not breeding?

3 weeks ago i bought four convicts from a fish store. Soon, i seperated two males and left my main tank (20 gal high) with an adult male and female (orange colored belly). I feed them frozen blood worms and put RO water into the tank but they still haven't bred, even though there are two rocks, a big seashell, and an artificial plant in the tank. Please tell me why they aren't breeding and how i can get them to breed