Female Guppies can hold their fry until they feel water conditions are right for survival of the fry. Large, weekly water changes with new, treated tap water that's a bit warmer than that in the tank, will encourage the female to drop the fry. If you don't have a lot of floating plants in the tank, then get some Common water weed (Anacharis) from the local pet store. It's cheap and the stems can be dropped into the tank. The stems grow quickly and provide hiding places for the fry. The jungle like effect will also make the fish feel more comfortable.
"Fear not, my young apprentice. Just change the tank water."