Brilliant! (as they say in Britcoms) Thanks, jford, that was the problem. And, not to belabor the point, but in case anyone has something similar, everyone's logic (except maybe RB's) was flawless. The Scarlett Solo has the mic input on the left and the instrument input on the right. So, it records to those channels discretely, in case you're recording 2 channels live. But the default Real Band setup only records the left channel when recording mono. So of course my mic recorded and my guitar didn't.

In the linked picture, Track 1 is my recording of the guitar -- track set to mono, like a good boy, but gold background, nothing recorded. Track 2 is recording the microphone with the track set to mono, but everything working fine. After I read your post, I tried recording the guitar in stereo and, as you can see, it came in on the right channel.

I haven't tried recording again, but as you can also see, I found the setting in the Audio Drivers window that uses both left and right channels as the input for mono recordings. (If it were up to me, I'd make this the default, rather than the left channel.)

Anyway, thanks again, all (and I, too, am glad your situation is comfortable enough to have checked the PG boards!)

Bill

https://www.dropbox.com/s/ll7s9b5i42kub8h/RB%20Audio%20Drivers.jpg?dl=0