Dura Ace 7850 spokes

[denvervig]

We have an Ultegra wheelset on my son's bike and the 20H Ultegra 6700 rear hub is toast and so we found a mint condition Dura Ace 7850 with all of the spokes except for one as the replacement. The plan is to re-use one of the spokes attached to the Ultegra hub but the threaded hub-end of the spokes do not want to unscrew. Are these screwed on with loctite or equivalent? The threaded hub ends that came with the replacement hub screw/unscrew without any issue. I must be missing something obvious!