For dealing with recalcitrant nipples, here's a technique that has worked for me:
The wheel must be securely installed in a truing stand of considerable strength, though your bicycle's frame and fork would also likely do. Strangely enough, the standard Park shop truing stand, TS-2, does not hold the axle tight enough.
With the axle solidly supported, pull the rim over toward the flange where the troublesome nipple is. Pull on the rim right adjacent to that spoke. This will greatly reduce the tension on the spoke. This often does the trick.
Sheldon "Bend It Like Brown" Brown
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| I reckon there's more things told than are true, |
| And more things true than are told. |
| --Rudyard Kipling |
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