I have always used the TV Listings web page (http://xfinitytv.comcast.net/tv-listings) to search for shows and send the "watch channel" command to my main DVR TV.
I have four boxes and recently only three of the four show in the "select a device listing". The Main DVR no longer shows as a device on the TV Listings page.
All four devices do show in the watch on TV on Demand, Movies on Demand and Featured on Demand pages, the problem is only on the TV Listings web page