Try a drop of triflow where the spoke leaves the nipple, and then loosen the nipple to get more into the threads. Of course, if the nipple rounds off, replace it. Lube each nipple and give it a little time to penetrate the threads, then see if you can loosen each nipple ¼ or ½ turn. One more drop of triflow after they've been loosened and you should be able to true and tension the wheel without fuss.

I recently had a set of nipples on a 27" wheelset that wouldn't budge, along with a few that had corroded and failed, probably due to road salt. In trying to free a few of the spokes I instead broke them at the nipple. Since I loosened the spokes all the way around, the wheel needed tensioning and dishing after I replaced the broken spokes.

This was a 1970's wheelset so I'm not sure if the spokes were stainless.