There are a couple things you could try before giving up. Try lightly misting the tub and both snakes when you introduce them. Try putting a fresh shed from another male in the tub with the pair (if you don't have a fresh one, get an older one (if you keep them) and moisten it just a bit). You could try popping one of your other males (safely!) and if you get some sperm plugs, rubbing that on the neck/back of the female. You could try putting another male in with the female for 20 or so mins (keep and eye on them-no locking!) and let them scent mark around and then putting the intended male in.

Essentially what you're trying to do is trick your intended male into thinking there's some major competition for this female. I wouldn't go so far as to put 2 males in with 1 female, as that can have disastrous consequences.

Have you seen any mating behavior from either of them after introduction? Sometimes if you sit quietly after introducing them, they will begin their courtship and both will scent the enclosure. The female will sometimes wag her tail (it's the funniest thing...). Usually there's quite a bit of urination.

I would try each one of these a couple times before moving to the next/giving up.