At six months he should be raring to go; he may not know which end is the operative one, but he should be trying hard! When I have a buck that behaves as you're describing, my first suspicion is that he's uncomfortable somehow. I'd check his vent area and hocks.
If he has a cut or pimple or is otherwise sore around his vent area, he'll go to mount then change his mind when it hurts. Similarly, if he has sore hocks, as soon as he starts pressing down on them he'll want off, although usually that's more obvious as he'll generally stop and lick his hocks. Also check him for split penis - sometimes when it's a severe case the buck won't breed.
If he is out of condition in any way, that can also diminish his interest. Make sure he's at a proper weight and in good health. When I don't see any obvious injury, I usually try worming the rabbit. I'm not sure if Fenbedazole (Safeguard for horses) actually kills worms in rabbits, but whatever it does, it can bring rabbits that just aren't quite right around, when nothing else I've done for them has helped.
I agree with
@RabbitsOfTheCreek that it would be risky to leave them unsupervised. Even if they don't actually hurt each other, if the doe gets frustrated with his lack of action and becomes aggressive, she can ruin him - he may end up afraid of does and unwilling to breed.
Also, you will not be able to know for sure whether or not she's been bred, so you'll have to wait a month to find out, which is a pretty significant waste of time for anyone trying to raise meat or show rabbits, which I'm assuming you're doing since you've got NZs.