5.1 Channels only have sound from Left and right.

If you've got a 5.1 multi-track session and dragged a 5.1 channel source file to it, it's down-mixing to stereo by some completely backwards default that I can't begin to comprehend.   Create a new 5.1 track within the session and place the file there and you'll get all channels output.   It'll show all channels being mapped to surround outputs within that track but it's still downmixing it to stereo.  

If you're just in the file editor for a single source file I can't help you there, although I had an issue where ffmpeg-produced 5.1 FLAC files would show as 6 channel and their type had to be converted manually to 5.1 (no other options, just change it to 5.1) before they'd play back properly as well.  This seems to be ffmpeg specific as it, in turn, can't properly read the 5.1 channel flacs saved out by audition afterwards.