There may be two clear plastic optical eyes on either side of the bill box.
What happens is the head shines infrared light down into one side of the bill box.
The emitted light goes around through a clear plastic molded channel inside the bill box and comes out the other side back up into the head into a receiver optic to verify that the bill box is in position....you need to get the dust off of the ends.
Also, if there's dust on a "stacker leg" inside the bill box, the infrared light doesn't get thu to the other side, and the head throws out all sorts of errors.
This "stacker leg" is a clear plastic post...looks like a triangular-looking stick in between both ends of the infrared optical channel plastics...you have to get the dust off of it.
Some guys actually throw the plastic bill boxes into a washing sink and spray hose the crap outta them...others put them in a dishwasher, while others know how to point the straw of a compressed air can onto the "prism leg" to get the dust off.
I use Q-tips with a long foreceps to reach down and rub the dust off of the prism leg.
Do not use any cleaner that contains alcohol....the alcohol actually degrades the plastic optics blurry over time, making it harder for the infrared light to go thru.
Plain soapy warm water, not hot, and a clean rinse is the way to go.