Photos not being scanned for people in Catalina

I'm a first-time Mac user, running Catalina (10.15.3), and I can't figure out how to get Photos to scan my photos for people. When I click on "People", I get the following message:

0 Photos Scanned

Photos will continue scanning your remaining 5,547 photos when you're not using the app and your Mac is connected to power.

I have left the Mac powered on for over a month (aside from an occasional reboot) and have tried leaving Photos running, closing Photos, etc., and no change in behavior.

All the photos have been imported directly to this computer from my various Apple devices (iPhones, iPods, etc.), and are stored on a 5TB external hard drive that is permanently connected to the Mac via a USB Micro-B to USB-C cable. The drive is formatted with exFAT, and I'm wondering if this could be the problem, but don't have another empty drive to experiment with (and the internal drive is too small to use for this purpose). Is this a known limitation of photos? (All other Photos functions seem to work, and the other media stored on the drive seems to be fully accessible through Music, TV, etc.)

Any suggestions would be appreciated.