Why wont my firemouth cichlid grow?

i have a 55 gallon tank. i have 6 convicts, 2 jacks, a firemouth and a jewel. i've also had in the past a red devil, a green terror and a yellow lab in the same tank. i know i know usually mixing south americans and africans isnt ideal but it seems to have worked out for me. and it seems like the tank being "overcrowded" hasnt been an issue either. i got my jewel and firemouth at the same time and same size.(about 2 inches) the jewel now is about 4 inches and the firemouth is the same. 2 of my albino convicts grew a little, and some grew to full lenth and are developing big knots on there heads. that goes for the jacks and the other fished i used to have. they all grew just the firemouth isnt growing at all. ive had these fish or about 7 months now and dont understand. does anybody have an idea what could be going on?