The assumption was that the light shield was doing its job. This sort of thing is one of the hardest problems to train yourself for. You can't go into troubleshooting something with the mindset that nothing works right and the whole things was built by monkeys. This also reminds me of one of the things I got spoken to about when I was working at a start up aerospace firm. I spent time documenting my work and even going as far as writing up a 'theory of operation' for some of my more clever designs. A freshly minted COO with no technical experience had words (email really) about all the time I spent not designing stuff. He failed to remember that the documentation that I did was a big contributor to a million dollar aerospace prize and also received praise from the US Air Force when we were working on getting permission to fly rockets from Cape Canaveral. (this is not SpaceX). What I'm vaguely getting to is developing the habit of looking at things from first principles. If you didn't design it yourself, a theory of operation document can go a long way to understanding a device, circuit or system. It could even be more valuable than a diagram or schematic since those can get out of date due to modifications, but the basics of what being done stays the same. Knowing how a scintillation counter works in simple terms could give insight into the sorts of things you need to check.