Mollie fry are larger than guppy fry and the largely vegetarian mollies are well known for leaving their fry alone. Those fry may be too large for the guppies to take.

Oh! Are your female guppies over three months of age and an inch long?

Sometimes a feeding of live or defrosted and rinse frozen foods will help. I have especially had luck with "worm bowls" in the tanks of livebearers well-known for cannibalism. See Food in General and Live Foods.

Another, nastier sounding, but treatable explanation is that your guppies have internal parasites, probably some sort of worm infestation. This is especially likely if your females are large and they have left no fry. Treat them and the tank with an anti-parasite application. Your LFS (Live Fish Shop) should have at least one available for internal parasites.

For a lot more on this, go to the Immediate Help links and check out maladies like Camallanus or Capillaria. One can get very involved with medicines called anthelmintics. But there are several commercial combos which will work.

I even picked up gel-tec at Petsmart and it works well for what I was looking for. The young man working the department was unaware of it's existence. Be prepared to thank whoever you ask and then do a little label reading. ;)

Please tell us if you are able to identify the cause of the problem.