Well I was just wondering if sometimes it reads and sometimes it does not because something may not be adjusted correctly.
Try making some very small turns to any of the black potentiometers to see if it helps things. Just try one at a time and make sure to remember where it was before you started in case you need to turn it back. I would start with the black pot closest to the two whote pots.
Leave the white potentiometers alone. Generally those do not need to be touched.