We have 2 sites trying to connect with eachother here at the University of Minnesota. One site has a VSX8000 and the other has a Tandberg C20 with software version 3.1. The site with the Tandberg is able to see the camera source coming from the Polycom, however, the Polycom cannot see the camera video coming from the Tandberg codec (Tandberg 1080p camera). We have called both into a bridge which ended up working just fine, but this issue remains while trying to call point-to-point.
Trying putting your HDX into Basic Mode under Admin settings>Network>Call Preference and see if that works, I have a feeling that the polycom and tandberg might have conflicting protocols at 1080p. Putting your HDX into basic mode will use Standard protocols for the HDX.