I've had some complaints about the 'recent conversations' not showing everything it should.
It's not clear - for a start - how 'recent' is defined. Having dug into the code I find an unreadable SQL query (why-oh-why etc.) that doesn't help at all. However, I can see that it should be displaying messages from the 'mdl_message_read' table but it seems to be displaying only some 'random' selection of the messages in the table (even taking into consideration multiple messages from the same user - it should only show the most recent).