Thanks for your suggestion, JS. I am familiar with Calibre (using it mostly for Alf, for backups), but since I've always been able to get books to my Nook Simple Touch using a PC file manager (and since the doc says you can), I was surprised to be having such trouble with the Kobo.
However, I have discovered the problem - you must use the ROOT directory of the external sd card and/or the built-in storage. I had been copying my epubs into a kobo subdirectory (probably should have realized not to fool with it!). Once I put my files in the root of either storage place, the device immediately noticed the change and updated the library (didn't even have to power off). So, it's looking really fine now.
