Most of them that get stuck real bad likely due to the engine sitting for many years without running. Rust never sleeps. Running the engine on a regular basis forces these parts to continuously go through heating and cooling cycles which keeps the shaft from seizing up.
Another cause is when the bimetallic spring or the pin which retains it to the manifold rusts away. The hot/cold cycling continues, but the movement does not. That is when carbon in the exhaust is no longer your friend.
