Don*t go with the "clean reinstall" quite yet. Is you Gmail account configured as an IMAP account? If yes, it is using a feature called "condstore" which is supposed to better coordinate activities when the account is accessed from multiple locations. It is known to be buggy though on Thunderbird*s side and may miss incoming mails in this way.
The workaround is to disable that feature:
1. On Windows, enter the Options window from either the ≡ application button or the Tools menu;
2. Go into the Advanced tab and find the "Config Editor" in its General tab;
3. Enter condstore into the search bar on top of the about:config window;
4. Double-click on all mail.server.something.use_condstore entries to toggle them to "false" (as a minimum, this should be the "default" entry;
restart Thunderbird and see if it fixes the problem.
https://support.mozilla.org/en-US/questions/991323
The workaround is to disable that feature:
1. On Windows, enter the Options window from either the ≡ application button or the Tools menu;
2. Go into the Advanced tab and find the "Config Editor" in its General tab;
3. Enter condstore into the search bar on top of the about:config window;
4. Double-click on all mail.server.something.use_condstore entries to toggle them to "false" (as a minimum, this should be the "default" entry;
restart Thunderbird and see if it fixes the problem.
https://support.mozilla.org/en-US/questions/991323
