Male dog is being aggressive

Question
QUESTION: Hi.
 I have a 6 year old German Shepherd bitch named Maggie. Right now we are worried that she is getting too old to breed, so we're trying to breed her one last time before getting her spayed.
 Our friends have a 4 year old stud named Wolfgang, but, he's being very aggressive towards Maggie. After a week of almost constantly having them around each other, he still has the same reaction. Barking and biting. The vet says she is ready, but Wolf isn't interested in anything other than biting her. A couple of times he sniffed at her but other than that, there's been little to no progress.
 What could the problem be?

ANSWER: Hi Dante

Is Wolfgang aggressive to other dogs or just to Maggie? My question to you would be why would you want to breed with a male with temperament issues?  Is that something you want to pass down to pups?

Has he ever bred before? Is she flagging?  How far along in the heat cycle is she?

Regardless, 6 is getting old for a female to breed. A female is born with all the eggs she is ever going to produce, so at 6 her eggs may not be prime.  How many times has she been bred? What were her litter sizes? Did the vet do a progesterone test or smear?



---------- FOLLOW-UP ----------

QUESTION: He doesn't have temperament issues. He's great with people and the other dogs at the house.
She has been bred before. Shes had 4 litters of pups so far with about 6-8 pups in each litter. Shes on her 9th day of her cycle right now.
I dont think she did any tests on her since we were just taking her to get her groomed.

Answer
Has she been bred back to back, was she bred young or did she have a litter every year?

I only ask because each litter takes a lot from a female and as a female ages, it's harder and harder to bounce back.

Day 9 is still early, they should just be introduced now with her coming in the next few days.

Good luck