Your question is Customer-Site Hardware Troubleshooting. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
How do you troubleshoot hardware that works during in-office testing but not at the customer site? An example diagram was drawn on a board showing how the device is set-up to work.