Breeding Problem w/Golden Retriever

Question
I have a 3 year old female Golden. I  have attempted to breed her three times and only ever achieved one (large & very healthy, wonderful puppy). She was a great mom.  The first breeding resulted in one tie, then during that heat a tie was never achieved again, with the male mounting, trying and trying but unable to penetrate. She stood patiently flagging the entire time. Two more heats have passed since the singleton  pup and both times we have been unsucessful in attempts at a litter- the male can never penetrate (I've tried 3 different, proven studs). Any suggestions. Sorry for the lengthy question.

Answer
I would take her to the vet and have her checked for a stricture in her vagina. It's kind of like scar tissue. It can cause what you describe. Usually though, after a dog whelps a litter naturally, she would be able to have a normal tie. Unless she experienced trauma to her vagina or maybe an infection after her first litter. I would definitely have her checked for this. It is something that may be surgically corrected, or you may have to decide to use AI with her next time. Good luck!