Thanks for using the Answers Forum! In a situation where you are getting audio but no video when you go into HD streaming there are a few possibilities to what could be going on. I would recommend checking out the links that I post below. These are usually what we cover when we see only audio coming through on the higher resolutions.
Also, just make sure the cables that you are using are able to transmit HD signal. Here is a link listing the different cabling types and what they can carry:
Hope this helps,
The Slingbox Moderation Team