i just finished lacing up a wheel and i'm having some issues taking out the spoke slack. i'm trying to tighten the nipples but they're only threded on a 1/4 of the way and they wont thred anymore, the spokes are just starting to twist.
here is a picture of one of the threads, i can't get it past where you see it. when i try to thread it more, the spokes start to twist
Definitely sounds like you got 15ga nipples with 14ga spokes. The SpokePrep or Loctite will prevent the nipples from rattling loose, but they don't bind so much that you end up twisting the spokes when trying to roll the nipples over.
As a test, take out one of the spokes and scrub off the SpokePrep with a 3M Scotch-Brite pad (kitchen-sponge with green scrubber). Then try threading the nipple on again. Will at least rule out or confirm that the SpokePrep stuff is causing you the problems. Or you can also try threading those nipples on some spare 14ga spokes with clean threads.