Slow motion movie
The lever on the side of the type head which moves down when the ball should rotate by 180° to the side with the upper-case characters:
Pictures taken from the movie
I think this is where the problem appears: the lever should be in the lower position at this time in order to prevent the movement of the black spring. Only if the spring is in its original position the typeball rotates by 180°, if the spring is pushed out the rotation is blocked. The workaround mentioned in my typecast consists of manually moving the lever down and keeping it there until the typeball has rotated. This allows to type upper-case characters.