have you checked the option in account settings specifying that the detected emails be moved? the default is to not move them. You have to turn the option on.

Yes, I have the option set.

Still having the problem. Everything was fine until a couple of months ago.

what anti virus are you using. They are notorious for causing mail moves for any reason to fail while they scan. It is for this reason you find people recommending exclusions for the Thunderbird profile folder in security software.

I use Norton 360. How can I exclude Thunderbird? Thanks, Les

I suggest you look in the Norton instructions. I don't use Norton, nor have I willingly touched in for around 20 years.

Their instructions are here. https://support.norton.com/sp/en/au/home/current/solutions/v3672136

Details on where the profile folder will be found are here https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data?redirectlocale=en-US&redirectslug=profiles-tb#w_where-is-my-profile-stored