I know that reproduction ain't always that easy with these fish. I'm not keeping them anymore. But when I tried to get them to breed I always put more colder water in the tank. They didn't start breeding for just over 6 months before they started mating and have a small batch of fry. Adding colder water did the job at my place. But there are more livebearers that don't breed that easy.
I know some experienced fish keepers finding it very difficult to get them breeding and others with no success . I personally have great success with this species what works for me is a change in their diet , last week I got 25 babies from a single female which seems to be about average for them , I kept 10 from last year's brood all doing well it probably won't be too long before they start to breed