The web interface maintains it's own internal cached folder list separate to the IMAP server. This list is only re-synced when you go to the Folders screen. This could be confusing because if you use an IMAP client to create a new folder, that folder will only appear in the web-interface if you go to the Folders screen to force a re-sync, even the 'Account problems' link would not fix this. This has been changed so that running the 'Account problems' link tests will force a re-sync. Going to the Folders screen is still the recommended way to discover new IMAP folders in the web-interface however.