Are you sure you pressed ACE's [SAVE] button in the right way? Sometimes is necessary to press several times the button to save the new payload.

I suggest to check [FUEL AND PAYLOAD] before press [FLY NOW].

Sometimes I have the same problem and when it happens I change the flight number at [CURRENT AIRCRAFT], then I check if the new payload is ok, and only after I close the ACE and go to [FLY NOW].