Sexually mature and optimum weight are really 2 different things.

Rats become sexually mature at 5/6 weeks of age however weight wise ideally you want to wait till the females are about 200/250 grams.

Did you get that group at that size? If you did it is possible that your group is already too old, either the male, the females or both which is why I always encourage people to get small rats around 4 to 5 weeks and raise them up to size.