I'm trying to apply a systemwide upgrade from a co-worker to my M-Class economy fare. My outbound flight got automatically upgraded, however, my return flight doesn't even give me an option to apply the system wide. I doublechecked the receipt and it's M class all the way.

Flight in question is NRT to IAH UA6 on 13 Feb.

Any tips on what I should say when I call up the help desk to get this resolved?
Any help would be appreciated.
This is all it says under Fare Rules in my receipt:
REFUNDABLE/CXLFEE/CHGFEE