Check for a bent PTO shaft. Raise wing all the way up & secure it. Unhook shaft, then let let wing down. If it goes all the way down, then the shat was holding it up. My dad had loaned out his mower & it came back & would not lower.
I used to have an older batwing that would do that. I had to get off the tractor and give it a push or sometimes I could take off and make a sharp turn with the upright wing to the outside and knock it down that way.