calendar import sets up but doesn't initiate

I have a shared family calendar and a private google account calendar that is separate. I tried importing some events from the family calendar to the private calendar. I created a .ics file, and started using the calendar import function in Thunderbird on my Ubuntu laptop. I found the file, selected it, and filtered for current and future events. I then clicked the confirm import button, and the screen changed to a view progress screen. This screen correctly shows the target account, the source file, the import type, and the number of events, so I think it's correctly set up. The progress bar shows "Importing... 0%", and there it appears to stall, consistently after several attempts. I saw another thread that talked about read-only calendars, but I don't see a setting, either at the Thunderbird level or at the calendar level that might be restrictive in that sense. Is there a preparatory step or a setting I'm overlooking?