How old are they did you purchase young rats and raised them up or did you purchased adult?

Reason I am asking is that if you purchased adults the female could very well be an older female at the end or her reproduction life.

Could be other things

Do you have more than 1 male and one female?

If you only have 1.1 your male could be unfertile.

They could be too young.

Etc there can be various reason why they have not produced yet, since you do not give any details it's hard to tell.

I can tell you that I usually have babies within 3 to 4 weeks once paired even with first time breeders.