This happened to me.
Took 30 mins with a phone agent and another 20 mins with the web phone agent.
The issue for me was that even though the original ticket included non-UA flights essentially the FFC can only be used for UA flights. Rebooked today and have FFC left over..
The FFC is supposed to be available for preferred airlines like LH but it just was not working for me..
Good luck rebooking.