virtualbox.org

This ASUS Xtion depth camera works fine with my Linux host (and my dual-booted windows install).

In my Win 8.1 VM, it gets detected and some initial connectivity is established (it goes into an active state) but no data seems to be received from the camera itself.

Googling doesn't turn up much. Yes I'm using the proprietary extensions to enable USB passthrough. Any ideas, please?

Cameras have their own special mode for passthrough, they don't use/work with the standard USB passthrough feature. Take a look at Chapter 9.7 Webcam passthrough.