Do you still maintain the same water change routine used when it was small?
I have found that as cichlids grow, water changes should be increased.
Obviously, as cichlids grow they put out more waste, and pheromones bringing nitrates higher, and maybe producing pheromones or hormones which can slow growth, so if you have maintained (say) 1x 50% water change per week, it may be time (now that the mota is larger and putting out exponentially more waste) to up to to 2x 50% changes per week (as an example).