Hello zzz Top, There are a number of reasons why people don't have succesful matings. It could be that it's an over-familiarity thing, but you have not described what happens when you want them to mate. Does she refuse him, is he not interested, are they trying but not succeeding? If she's had a litter to him already, there shouldn't be anything physically wrong, so I wouldn't rush off to a vet just yet. Do you check her for when she is peaking? Can you give me more info and I can probably point you in the right direction unless there is something physically wrong.
Cheers, Elsha.