Either the male could be too young or infertile, or the females could be too young. It could be any number of things. I'd say just give it some time longer before scrapping them and starting over.
It sounds as though the husbandry isn't an issue. Did you acquire these mice as "pet mice" from a pet store, or from the feeder bin? Feeder bin mice usually are there for a reason, and any I've bought in the past never proved to be a healthy addition and were culled shortly afterwards.
Depending on the age of the mice and the introduction, I've never really had it take longer than 6 weeks for the females to show signs of pregnancy. Usually they've gotten their pecking order established, the male isn't too hen-pecked, and the ball starts rolling.
The cage sounds huge in my honest opinion, so that could be the problem. Limit anything that would otherwise distract horny mice, such as wheels, toys etc until the babies start popping out. Make themselves the only source of exercise. If you can, cut back the cage to a 10 gallon for a month or so to see if that helps matters any. Mice from my experience tend to do better when cramped, from our perspective of being cramped. Once they get used to the idea of what they're supposed to be doing, then you can add things back in.
Let us know how it all works out. =)
