Can't delete mailboxes

This is odd, but I can't delete some mailboxes. I'm sorting and cleaning things up and when I delete these mailboxes I get a pop-up to confirm deletion, I say OK, the mailbox in the tree view closes, but it's still there. I've tried closing the program and restarting it but it's still there. Am I missing a step somewhere?

Thanks!

When you delete a mailbox, all messages from that mailbox are transferred to Trash first. Mailbox is actually deleted only after all messages were transferred to Trash.
You can bring Tasks pane to front (View/Tasks), to monitor deletion progress (when you are dealing with larger mailboxes).

If mailbox was not removed, and the task is finished, open the mailbox - inspect remaining messages. Check also Log for any errors/messages (View/Log).

Nothing in the log. And no emails in the folder. Yet each time I try to delete it, nothing happens. the folder remains.

So for fun I renamed of the folder. It changed the name. And recreated the folder.

In that case just open corresponding folder in File Explorer and make sure there are no third party files/folders inside (backups, files copied accidentally etc).

Normally, mailbox folders (.mf) contain just other mailbox folders and mailbox files (mbc/mbi pairs), nothing else.

There was nothing in those folders that shouldn't be there. Just mbc/mbi pairs. So I just manually deleted the offending .mf folders while Pandora was closed. Opening the program, it looks like everything is fine so I guess I didn't do any harm. Still wondering what caused it to happen.

Pandora is very cautious when dealing with files and folders. It also uses plain vanilla Windows-API when possible, so if deletion was blocked, it will flag the mailbox.
To work around the issue:
1. Right-click on mailbox and click Compact.
2. Delete the mailbox again.