It may be that the folder is not 'subscribed' in the account, but is in the unified view. Verify that by highlighting account in non-unified view, rightclick ad select 'subscribe'

It was subscribed. When I restart Thunderbird, I often only see the inbox and no other subscribed folders

Your first post discussed the Sent folder, but now you state you just see the Inbox. If we're discussing the unified folders, you might try this: - click help>troubleshootinginformation - scroll down to 'profile folder' and click open folder' - exit thunderbird - click the Mail folder and delete the 'smart mailboxes' folder - restart thunderbird and it will reconstruct the Unified folders - then, if unified view is incomplete, recheck the subscribe status for the accounts.

Gmail automatically copies sent messages to the Sent Mail folder, so 'Place a copy in' in the Copies & Folders section of Account Settings should be unchecked, and the Sent Mail folder subscribed.