Sounds silly, but are they big white mice? I have a line of big (when they finally finish growing) white mice that I got from a NJ supplier. These mice seem to take forever to settle in and start to breed. But with patience they take off and like right now I have a 1.5 group with over fifty peachfuzz and two does ready to pop again. There is/was also a line of black and white mice that did the same thing. I would say that if they look good otherwise I would leave them a bit longer.
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Sonya

I'm not mean. You're just a sissy.
Happy Bunny