Call back.

Could be a couple of things. Flight 863 is listed as FCY, but we all know it's just FY. However, NC = 0 and a less informed agent may think you're only entitled to upgrade to C from Y. But this flight has no C, only F. I think if you get waitlisted on this flight (in F) you will clear instantly. You just need an agent to get you on the NF (or X instead of I).

If that's not it, then possibly you are booked on 863 from another city and the pair of flights don't have F (or C).

Billy