I had a similar issue in my industrial automation class. We were sorting cylinders by diameter and height as they went down the conveyor belt. PLC controlling motors, sensors, etc.

My group got everything setup, built our program, and everything worked fine. Waited a few minutes for the TA to verify, but it failed. We changed a few things, it worked, but failed when he came over.

Another group looked over our code, no issues noticed.

Finally I realized I was standing when we were testing things. I sat down waiting for the TA to verify. My shadow blocked the sun from the photo eye. Wasted half the lab on an issue that was entirely dependent on our position in the room, but found the root cause.