Livebearing females can hold off dropping their fry until they feel conditions are right for their fry to survive. Pure water conditions and the right foods will help. Fish need at least half their tank water changed weekly. This will guarantee a steady water chemistry. Also, a pregnant female is stressed. Add a little standard aquarium salt to the new, treated tap water. A teaspoon in every five gallons of water is about right. Salt has a calming affect on fish. And, make the new water a little bit warmer. I would also add some floating plants to the tank. This will allow privacy for the female and a hiding place for the fry. I would start feeding a little Spirulina every couple of days. This food is high nutrient.
"Fear not, my young apprentice. Just change the tank water."