The Rebels have lost a bit of structure on the wings the last few weeks. If you look at the Chiefs and the Blues game, at least half of the tries scored against the Rebels was out wide.
Vuna and Inman in particular are being sucked in and leaving a couple of opposition blokes out wide for quick ball. The Rebels just don't react quickly enough to cover those wings.
Vuna used to sit so far out wide he was standing outside the field of play. Recently he is now standing almost midfield!
Inman missed a few against the Blues for this very reason; I assume he either didn't know who he was covering, or was trying to second guess who Vuna was covering. Just poor structure and specifically, communication.
Having said that, Mitch's workrate has been immense recently.