To anyone who might come by here, I had the same problem but found that the channel was sending to an auxiliary strip. I simply clicked to make the aux-strip mono and then back to stereo and voila. Perhaps it could help someone!
I just encountered this problem when having four trumpets sent to one Bus. When I panned the tracks individually, nothing happened. Then I turned off stereo (= set it to mono) on the bus and back again.. Voila.