Hi Larry;
The belt appears good, with no cracks or squeals or apparent slipperyness.
I presume it's tensioned properly (everything else that runs off of it seems to be working properly, and no squeals or noises), but can't say for sure.

And like I said, I can rotate the compressor (not the belt driven outer ring) by hand, so surely don't believe it's locked-up.....

I'll try another 12v source on the clutch tonight.
I'm wondering if I just wasn't using enough current on the clutch....
Mechanically, there just doesn't seem to be any reason the clutch won't engage and stay engaged.

I'll post back. Thanks.

__________________
Barry