Age alone isn't the primary factor in breeding. Just because the female has hit a certain age doesn't mean she's ready. We've got a female that just turned 2 in June, and she's been refusing our big male Arnie (who turns 2 in October). He does the same thing as your males. He does the head bobbing, grabs hold of her neck, then just lays there.
I've spoken to a couple of big breeders, and they think it's simply due to the female not being responsive to the male. Either because she doesn't feel ready to breed, or she just doesn't like the male. Either way, the male isn't going to force the issue, and that's as far as it goes.
Last year when you brumated them, did you brumate the female too? When did she go to sleep and when did she wake up?
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Axe
The Reptile Rooms