Just a FYI. Don’t know if it is useful, but this is what happened.
I am first time user. Imported 3000+ emails, selecting several email boxes and using F1. Everything seemed to have been imported, but (with no activity going on, and “anywhere” and “partial word” selected), it wouldn’t find anything.
I spent about 15 minutes trying to get it to work, then closed the program and restarted it. Then it worked as it (apparently) should, finding anything I searched for.