My hamster wont get pregnant?

I have two hamsters, both 10months old (definately a male and a female) I've had them since they were both a month old. I house them in the same cage and they get along fine. I've seen them mating a million times since they were about 3-4 months old but they havent had any babies. The male has been mated with another female and isn't sterile, the female ovulates every 4 days (thats why he sniffs her) but no babies. Any ideas why?