Actually I think I did figure it out! I still have Legacy installed, firewalled from the internet. It obviously doesn't sync, but I'm keeping it as a backup in case I need to check an older note at some point in the future that hasn't yet been converted to v.10 - and also as reassurance (and also to torture myself, since it reminds me every time I see it how much I preferred that UI!). I was using the same "Add to Evernote" folder for v.10 that I had been for Legacy, though I'd cleared it out. But it turns out that though Legacy is firewalled, it is still operating as a local app, so when I added a file to the folder, it was importing to Legacy! Legacy is much speedier with importing than v.10, so it beat out v.10 in the import 90% of the time, and once that happened, the note was considered imported by Evernote Prime - even though it actually wasn't now that Legacy is no longer connected to servers.
Anyway, once I set up a new Add to Evernote folder in a different location, the import to v.10 resumed working correctly.
I know there are others who have retained Legacy locally as I have, so this could possibly be helpful if anyone encounters the same issue!
