Manifold Heat Control Valve - stuck close

can somebody pls tell me how the manifold heat control valve operates in a SL-6/225 ci engine? My mhcv is "loose" and rotates freely when the engine is cold. Not too much "slop" either so I guess the bushings are OK in the exhaust manifold. Control arm rests to the right (closed) and rotates fully ccw when i ck it. It will return via spring if I release it from its "hot" position (open). However....

when I drive the car, the valve stays in the cold (closed) position. Refuses to rotate and plenty hot. My memories indicate the valve should open, meaning rotate fully ccw because of the bi-metallic spring. Am I correct? Where is my problem causing the valve to be "stuck" in the cold (closed) position.

Thx all, great forum. U guys have helped me w/ lights, headlight sw, faulty FUEL gauge, etc, etc. My car is a 1968 Dodge Dart 270 all original and just noticed the above dumb problem. Any guidance will help and fully appreciated