Guppies, and I think all livebearers for that matter, maybe not all, can hold sperm for up to six months. They may have not had a male and mated since you bought them, but they could have been "pre-hit" meaning they had already mated before you got them and have been holding the sperm. If you add a male to the mix, you will have fry. My guppies were a little odd. She didn't get pregnant for a few months, and I had one male and one female. I was new to fish and guppies, and took the advice of the fish store person, and I get now that it's supposed to be more females than that. My guppies would make love every hour on the hour, I kid you not. They aren't hard to breed at all. Just put a male and a couple females together, feed them, and you'll have fry.