Check with your email provider. Some providers do limit the number of hierarchy levels you can create.

Thanks, I'll check with them. But since I created these subfolders through their interface, and they obviously publish their existence to Thunderbird since I can subscribe to them, I'd be surprised if this was the solution.

I just verified that these subfolders are available through my phone, so it isn't a limitation on the provider side.

Can you confirm that you have this setting: either via Menu Bar: 'View' > 'Folders' > 'All or via Mail toolbar: 'Menu icon' > 'Folders' > 'All'

Yes, Folders All is selected.

Please check settings here: Right Click on imap mail account name in the Folder Pane and select: 'Subscribe' click on the 'Refresh' button then click on OK

Did this get a bigger subscribe list?

If no, then check this info: Some email providers let you control which webmail folders are visible in a IMAP account. For example, Options -> Mail Settings -> Labels controls that in Gmail webmail. The gear like icon in the upper right corner is the Options list box. Check that any missing or empty folders are enabled, if your email provider has that feature.

Logon to your webmail account via a browser to see if there is a means of controlling folders for imap.

If still no, please tell us some info: Right Click on imap mail account name in the Folder Pane and select : 'Settings' Select: 'Server Settings' for the imap mail account Click on 'Advanced' button

Is this selected: 'Server supports folders that contain sub-folders and messages'

I refreshed the subscribe list, and doublechecked the subscription for the folders in question, and they are subscribed.

I've seen the folders and their contents available in the email client on my phone, so they are being offered by my provider.

The Server supports folders etc... function is enabled.

Thanks for helping me get to the bottom of this, but we're not there yet, I'm afraid.

Does it make any difference if you open Tools/Account Settings/accountname/Synchronization & Storage/Message Synchronization/Advanced and check the folders for offline use?

All folders are already checked for offline use.

But in the mean time, the folders have appeared. I can't figure out how/when it got fixed, but it is. Thanks all for your help.