Floyd mentioned:
"The lights are installed at an angle so that the light beams meet at a point on the board surface to get the best reading."

Maybe the intersected beam-to-board distance isn't right? Should there be any gasket/spacer between the sensor mount and the truck?
This wouldn't answer the 'hand-pass-failure' issue, and the reading stops climbing after a certain crank-down point.....but still......?