It's entirely possible that you have a infertile male as you should have seen a litter at this point. If you've been doing this for "over 2 months" though you started when those females were way undersized for breeding but even then you should have seen somebody come up pregnant. I've had a couple of males in our colony that were duds when it came to breeding. I'd suggest trying a new male, keeping him with 2 of the females for 3 weeks and then the other 2 for 3 weeks and see how it goes.