Syrian hamster compatibility

[GeoHam1]

Dear community members,
I would value your advice. We have mated two adult hamsters, one 8 months old (the female) the other 3 months old (male) and no pregnancies Mating appears successful each time. They have been together at least four times, on different cycles, for 20 min periods of apparent mating. I am wondering what is going on 1) is the female grooming the sperm from herself perhaps? I have seen her groom afterwards. 2) they are very different looking hamsters. Might they be incompatible in some way? She is from show stock, he is not. 3) maybe this is just bad luck and one of them is infertile? Or perhaps her much bigger size is stopping him from managing it? I will attach photos.

Any advice gladly received!
Thank you.