1. Check your folder layout in Outlook and ensure your sent emails folder is selected for saving sent emails.
2. Check your Outlook rules settings and delete any rules that might be moving your sent emails to other folders.
3. Ensure the 'Save copies of messages in Sent Items folder' option is selected in the Save Sent Messages Options in Advanced E-mail Options.
4. Try disabling Outlook add-ins in case any of them are conflicting with your sent items folder.
5. Check the 'Show in IMAP' setting of Sent Items folder to make sure it is turned on.
6. Rebuild Outlook's data file to ensure it's not corrupted.
7. Check if the problem persists in another Outlook profile, or try in a different version of Outlook.