
He may need 'help' getting further forward on her! Oh boy do I know how much fun it is mating my main breed given their length of back for starters. We have to have the bitch across our knees (ideally with somebody up the front end to stop her moving around - assuming she's ready!!) and then help the dog get into the right place. If the dog is a maiden, he may even more need gentle assistance to hit the spot although some dogs back off completely if their owner tries to interfere! We had a 'rare bird', a male who preferred not to be helped and he could do it without anybody being involved - as I say, not usual with my breed!
And further, if your dog lives with her, it could be that the intention is there, but he's just a bit reluctant to really get on with the job, having been used to being seen off by her normally. It's usually best to remove the bitch from him as soon as she comes into season, only putting them together when you feel she's ready.
Do you know of somebody experienced (specifically in mating your breed) who you could call on to come and help - we had such a person and quite often would take our males to her, meeting the bitch and her owner there. She was brilliant and could mate tables to chairs!! It saved a lot of messing around, and the male getting exhausted. We'd give her a percentage of the stud fee - money well spent!