I got it working on one set of 3 drawings. Great.
Deleted those 3, and pulled in another 3.
Now the Drawing column showing the new 3 drawings to shift & trace shows them, but they’re grayed out, as are the icons.
WTF?
Well, I figured it out. S&T doesn’t work unless the drawings are selected with ONION SKIN enabled. I can’t fathom why this is necessary, as the act of choosing the drawings IMPLIES that I want to S&T!!! If anyone from TB is reading this, please for God’s sake automatically permit selected drawing to S&T without the strange requirement of activating Onion skin in the source layer. There is no reason for this; all it does is generate one more hoop for the (bedraggled) ex-Flash animator to stumble through. Thanks.