First, which guide are you trying to access? Are you trying to use the Guide from our Slingplayer app, or the Guide in the U-Verse box? What exactly goes wrong with the guide? Does it not pull up? Are there channels missing?
If the issue is with the Guide programmed in the Slingplayer, I would first try accessing that guide on the iPad, and toggling the "HD Only" option on and off, and see if that makes all your channels show up. Second, I would try reconfiguring the guide and the virtual remote by following the process in this article (even though you can control your TV):
If the issue is with the guide you access using the "Guide" button on your virtual TV remote, I would consider trying the AT&T U-Verse universal remotes. Entering "U-Verse" or "AT&T" in the search field when it asks for cable box manufacturer and model number (using the process from the article above) should bring them up.
Hope this helps!