I'm dealing with the same thing right now. I bought a 24 year old 5* AMC/AMR with CK special ability.

I've been playng him at both positions in every match I've had since buying and he's almost always rated as 5.

I've tried multiple times to sell, even postng at the lowest price and can't move him on. A this point, I may as well wait until the end of the season and try then.