livebearer problem

It should be pretty obvious when they're pregnant. They will get very plump and get a very dark spot just above their anus. It is possible that they're eating their babies quicker than you're seeing them, but you should at least notice their pregnancy.

Additionally, I know that when most livebearers get older they don't reproduce as easily as they do when they're young. Is is possible your females are just getting old?

Other things to think about:

1. Water quality - if it isn't good, they probably are too stressed to breed.
2. Comfort - Are there other more aggressive fish in the tank? Are there plenty of places to hide? (plants, caves, etc.)
3. If you have the space, maybe up the M/F ratio to something like 4 or 5 to 1. Spreads out the males tendency to be aggressive to the females over more females. Translates to less stress per female.

I find that my livebearers breed more in the summer, when the general temperature and humidity of the room is higher, and not just the tank temperature. Your fish are probably just having their winter break, I wouldnt worry about it. however, the points morleyz brought up are certainly worth considering.

Hope this helps,
Michael.

do livebearers need some salt in the water to be most comfortable? Does this qualify under water quality?

Certain livebearers, such as mollies and half beaks, do prefer a small amount of salt in the water. However, this does not mean they will not do well without it, and I for one do successfully keep and breed mollies in purely freshwater. Other livebearers such as guppies do sometimes have salt added to the water when being bred in fish farms to reduce the risk of disease, but this is not their natural conditions. As a whole livebearers do not require salt in the water to be comfortable, and those that would sometimes like a bit of salt to be added to the water can live comfortably without it.

Hope this helps,
Michael.