In my free Breath Tech experience on Oculus Home I've recently noticed that people sometimes leave optional $1 tips, but don't rate or review the game. It may be that people don't feel it necessary to review a game that's been out a while. Alternatively, it could be that the in-app purchase (tip) is in front of them while leaving a review is less apparent or requires more effort.
I believe that if we had an API call that allowed us to initiate a review / rating process from within our experience it could lead to more feedback being left. Another approach could be to start the review process by bringing up the dashboard and then selecting a prominent option to rate / review the game currently being played. The option could sit along side the [Quit] [ Resume] buttons that display over the game graphic.