I Upgraded a server from Ubuntu 16.04 to Ubuntu 18.04. This includes an upgrade from dovecot 2.2.22 to 2.2.33.2. 
Sometimes it happens, that dovecot stops showing new mail. There is no error in the log, dovecot just seems to do his normal operations but is not delivering new emails from /var/mail/user (for some users only) emails are delivered again if I delete index-files from .imap folder.

Ajay Guleria