True wheel has slack spoke, doesn't stay true

My first handbuilt set of wheels I built with the help of the LBS.
Without knowing what to choose, I went with DT Revolution spokes and
aluminum nipples to lace my Chorus hubs to Open Pro rims. The front
wheel has been fine and has only needed truing once to fix a few spokes
that had been left with some wind up in them. Since then it has stayed
true.

The rear wheel, however, has been a real pain. I have one spoke that is
one spoke away from the weld on the non-drive side that only has a few
kg of force on it. No matter what I do, if the other spokes are
tensioned properly and the wheel is round and true, this one spoke will
only have a little force on it. I can't see that the rim has anything
more than the usual amount of out of round near the seam and there is
no damage. It's been like this since new. This wheel never stays
perfectly true even through one ride, and I suspect that this plus the
aluminum nipples has something to do with this.

Should I just accept a bit out of roundness on that part of the wheel
to get some tension there, or are there any other suggestions?

Thanks,
Tad