No that's not the problem. Putting the wrong potentiometer in will just have the effect that most of the control seems to be over a restricted angle of the pot with very little change occurring over the rest of the travel.
Figure 1. Tracing the straight-through signal.
Finding the fault should be easy enough using a multimeter on continuity test.
- Test from 1 to 2.
- Test from 1 to 3.
- Check that you have continuity to 4 alternating off with each switch press.
- Test at 5 to check the loop integrity.
- 6 should also alternate on and off with the switch.
- Check at 7 and 8.
- Finally, there should be a ground connection to 9 but this may be done through the metal box.
Report back.