Seemingly randomly kmail checks email (auto or manual), progress bar completes but the new email do not appear in inbox.
I discovered this when after rebooting I would discover a backlog of several days worth of emails.
Quitting kmail, stopping and restarting akonadi and reopening kmail may help with those email appearing. Sometimes a reboot is the only way to fix this.
This is quite unnerving as I now can't trust kmail to tell me if I have new emails or not.
I have the same issue.
It seems the IMAP communication hangs in akonadi. Opening the accounts settings, opening the account settings window and then clicking OK will make the account to recover and downloads properly the new messages.
However, this is just a workaround is not a reliable way to use kmail.