TBP Not Reading the Sent Folder DAT File

I can see the sent emails in the DAT file if I open it with Notepad, but TB is only reading two of them, it seems to be ignoring the rest.

This happened in unified view. One minute the sent emails were there, the next minute they had all disappeared bar two.

I have five accounts: two POP3 and three IMAP. Is that a potential problem? I see that TBP has not created email folders for the IMAP accounts, yet it is still getting the data from somewhere. How does that happen?