you saw a thread about this topic. But chose not to share what you had already been made aware of through that thread. So do not complain when you have exactly the same introductory information shoved your way, or someone assume you know a lot more than you do and start assuming you know how things work when you do not.

How sub folders are downloaded depends a lot on the server implementation of IMAP. This is especially true of those using the Microsoft version and the google version, neither of which actually work as anything more than some sort of emulation of IMAP with special quirks for each implementation. I seem to recall something about Bugs in Exchange meaning only top level IMAP folder get synced or the top two levels for exchange/office360/ Hotmail etc. Google actually have server side settings for IMAP that can get in the way when you want full copie0 have you checked those if relevant? Then there is the Yahoo implementation that just stops working when the number of mails in the account exceeds 10,000. I suggest you offer explicit details of the provider, how long you have had the account in Thunderbird, the number of other accounts you have, how frequently you have Thunderbird checking mail for each account. Some folk invite disaster by setting this to one or two minutes and then seek support because it takes so long to complete it never actually does before it starts again.