What size your female guppy? If that small then she probably not be ready for bred, give her a more time and more growing. And sometimes there's no gravid on young females, can have babies but in low numbers. She's not infertile, she was just young, just wait till next month then she will have her first batch. It takes a time...plus the colors of this female (if its fancy) that means she's not ready. The true female guppy don't have colors its really normal than these fancy guppies....how can she be boring guppy? What kind is your male guppies? If they are fancy guppies, sometimes when they get older they can't do get their job done (unless you give them Viagra). Albino guppies can be sterile sometimes if its too much inbred.