i don't know if this is truth since Ive only seen it on this forum from 1 person, but i guess if a guppy doesn't breed early in its life it may not be able to breed ever.
also some fancy guppy types don't like to breed together for some reason. I'm going to find the link where i saw that . it goes more in depth as to hwy that is but i cant remember the article in detail. I'm sure a quick google search wold tell you more about problems with guppies breeding.
another possibility is that the strain you happen to have is just too inbred and isn't capable of breeding. that seems to happen once in a while with Fancy's. thats one of the reasons breeders will out cross with wild guppies. to help strengthening the strain.
EDIT: guppies may also abort if conditions aren't satisfactory.
it looks like she wants to breed but males think she is pregnat.can anyone help me? 
sorry, but these days you can never be sure. 
sama:
