Hello.
My RB67 120 back will stay red in the window where it is supposed to turn white when advancing the film. It used to go white upon pressing the small metal lever, but neither that nor winding a frame will make it act normal.
My camera is a Pro S, but i believe the back is a regular Pro.
(This is the of back i have)

What is wrong? (Or; optionally: What have i done wrong?)