To activate the Advance Unit (I), the 'normal' path to it has to get through a normally closed switch on 'O' and on 'BB'. (At least I think it's O; it's right on the crease on my schematic.)

There are two additional paths to 'I' when it's at the zero position and the 9th position, which are governed by the two switches riding on the cam.

But I'd start by checking those two normally closed switches for being clean and making contact.