So perhaps you have used a web-interface to create new subfolders on an IMAP server. The problem is that thunderbird doesn’t show them, and no amount of clicking on things seems to solve the problem or show these subfolders.
The answer is to click on the account in question (i.e. one level up from your Inbox), then click “Manage Folder Subscriptions”. From there on it should be obvious – find the folders, and tick the boxes. Job done.
Gravitystorm Blog 
Hi,
This probably means that your web client is slightly broken: If you’re not subscribed to the folder then it shouldn’t show up. Most clients will automatically subscribe you to the folder when you create it.
It took me a little while to realise that the subscription state is an account level thing rather than a client level thing.
Thanks for the Tip, I had exactly the problem and didn’t find the Subscriptions-Window. You saved me a lot of time 🙂