hello. My name is***** can help you with your question. If it's not printing from only one of the trees, it should be give you an error a bout a paper jam if this is the case then take sure that you've removed the back lower part of the printer and remove the bottom tray so you can see in there and see if there is something blocking the paper. Make sure there is nothing at all in the paper path as even very small pieces of paper can cause this to happen. Also remove and fan the paper making sure that it's loosely stacked in the paper tray. Please let me know if this helps or if you still have questions.
you're right in that it could be a sensor in the bottom tray. But, I would still go through the procedure I suggested as though there had been a paper jam. If there is a loose sensor, you will probably see it there. If you don't see a loose sensor then there may be a failed sensor that needs to be replaced. All it would take is a small piece of dust or something like that to block the sensor to cause it to malfunction. Often what I suggest doing is removing the paper in and lose parts and then turning it over giving it a little bit of a shake to make sure that nothing is blocking into the sensors.