Many things can stop them from spawning for up to several months . A move , any changes in the tank , not having same light cycles every day . Do you have your lights on a timer ? If not do so . Extending the daily light cycle is supposed to help get them in the mood . Feeding them live foods and feeding the same time every day is another trick . Increase temp to 80*F if not already , slowly lowering S.G to 1.017 , moon light , adding a good amount of baby brine shrimp for 3 days to simulate a plankton bloom[not too much , keep an eye on ammonia when doing this] , increase water flow , and adding some potassium permanganate to increase redox are some tips Joyce Wilkerson recommends .