Maybe the mapping of folders is not correct. If this is the case, then the following steps might be helpful: "Settings" > "Accounts" section > Tap the account you want to configure > "Folders" > "Sent folder" > Choose the folder you prefer > Press back (◀) button of Android phone.

Hi there! Tried it but it didn't resolve it. The folder was set to "Automatic (Sent)" and I changed it to "Sent".

UPDATE: I just deleted my account, uninstalled and reinstalled Thunderbird and everything seems to be working well again. Thanks to everyone for all the help! Much appreciated.

Same issue: K9-mail user since 2010, no config changes made in many months, if not years.

sent folder empty, all folders other than inbox, also empty. Inbox works. All other folders are empty, show 'Load 25 more', but won't fetch any messages.

Problem effects two accounts. Both IMAP, hosted on cPanel. A third account hosted on fastmail works fine.

Tried deleting, re-adding the account: no change.

Export settings, uninstall app, re-install app, import settings, re-enter passwords ..-

--  at first, appeared to still be broken ,,
--  but several minutes later, folders began populating.