Calumet C2 roll film back

I apologize if this question has been asked before; the search function in the forums yielded no results.

I have a Calumet C2 roll film back for 4x5 which is not working. The film counter is at 7, and when I press the little knob to release the film winding knob (which should then turn freely), it is not movable. An inspection of the outside and inside of the back reveals no obvious issues. Before I open it up and perhaps damage it irreparably, I'd like to ask if anyone has encountered this problem, and if there is a simple fix.

Thank you very much,

Raj