Breeding mice

So I'm making another attempt at breeding mice. Got a bigger cage bought 1 male and 1 female they don't seem to be breeding. I've had them for about 2 months. So I went and bought 2 more female and and 1 more male.
The other male is in a cage all on his own.
But the male I already had isn't Brothering with the 2 new females is it possible they are already pregnant?

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There could be several factors. The one male could be to young, to old, infertile. I've had times where they sometimes take a month or two to warm up to each other. It could also be temperature related. In the winter my production goes way down. Maybe try the other male to start with.