as said, make sure you have both males and females. a good ratio is at least two females to every male. guppies really don't need any help in the breeding department. chances are, like william said, that they had fry and you just missed it. check out the livebearer section for the pinned article on breeding. will tell you signs to look for when its time to put the mom in a breeder box/net. quite often, they ahve the babies overnight and eat whatever ones dont' hide fast enough, so you may never even know a birth occurred!!!!!!