permissions should be 644 or 444 owned by root.
if the permissions are too open, ssl/dovecot will refuse to load
them.
you may even see a message about it if you have verbose messages/
check your sys logs.
I had this problem once with certs that checked out fine, correct
< in dovcot config but didn't load.
chmod 644 /etc/ssl/certs/dovecot.cert /etc/ssl/private/dovecot.key
fixed the problem
regards, Tim