So it turned out to be a bad bulb after all. When I swapped out one of the non-working bulbs for a working bulb, I had the bad luck of swapping a good bulb for a good bulb. I simply got lucky while troubleshooting the rig after taking the reflectors off and loaded up four good bulbs into that middle set. After some more noodling, I figured out one of my actinics wouldn't fire up, and that was preventing the bulb paired with it from lighting up, either.

Note to self - when two neighboring bulbs fail to light, swap BOTH bulbs to confirm that the fixture is working, and go from there.