Let me start out by saying I have zero experience with shooting galleries, and I think I've already diagnosed the problem, but I thought I'd share so someone here could educate me more on these as they'd like me to help with their shooting gallery more in the future.
While working on some skeeballs at a local pumpkin patch they asked if I could give them any guidance on the intermittent gun issues with their shooting gallery. I've included some pics just for fun (god I love shooting galleries), not for help diagnosing. They have an 8 gun shooting gallery and 3 of the 8 weren't registering hits. I tried to intuit what might be the issue based on my basic knowledge of IR sensors and swapping guns to see if the problem persists. When swapping a non-working gun to a working position the gun still did not register hits. The guns would make sounds however. I should note all of the target sensors registered hits from good guns so it wasn't that we were just shooting bad targets. This lead me to believe either the IR senders in the guns were bad, or at least dirty, though they didn't appear to be. The gallery is in a barn area and is extremely dusty and dirty, so I suggested perhaps cleaning the IR emitters in the gun barrels before sending them in for service. They said last time they sent them in for service they came back still non functional, so they were hesitant to just send them back again.
Anyway, seems like a simple problem, but perhaps there's more to it. Would be interested to learn more about these.