Had that happen to a friend of mine, who was trying to swap with another crewmember. He called everybody, futures, scheduling, management, simulators, LMS training. Nobody could figure it out, it made no sense. Finally the resolution was that it was a trip that was going to another airplane, and he was going to get subbed on it.

Can you just pick up the trip on makeup and not try to swap for it? It may be hard to tell whether it's an issue with the trip you're on, or the one you're trying to get.