Outlook seems to have real problems "Synchronizing subscribed folders"! Yesterday about 14:00, I noticed that Outlook (on the PC) had not received any mails since 07:40, even though I remembered an email at about 08:45 (on Gmail on my phone). I went to webmail and there were dozens of emails not received including the one I had seen earlier. I noticed that it said it still had the "Send/Receive" progress in the status bar. I had to "Cancel all" tasks as the "Synchronizing subscribed folders" task was still running. Trying "Send/Receive All" resulted in the same thing. The only thing that worked was removing the account from outlook and re-adding it (which, of course, took ages because it had to retrieve all the IMAP folders again).
Today, it appears to be having the same synchronization issue, although the Inbox does not seem to be stuck yet!
Any ideas? (Apart from removing and re-adding the account)