[Dovecot] Dovecot don't recognise new messages in maildir /new

Hi,

i'm google for it and i searched the archives, but i can't find the problem i have or a hint for a solution.

I'm using Postfix as MTA, the new messages deliver by postfix to the maildir: /mail/domain.tld/userpart/new/

Dovecot sometimes show the new mails directly (it's very unusual) but mostly it needs for an hour that dovecot recognize the new messages.

Reconnecting of the client don't help. Tested with Thunderbird, Sylpheed-Claws, mutt and muttng.

Is there a bug known or do i something wrong?