Half of dash lights work?!?!?!

[HIGHWAY BY THE SEA]

A while back I decided to install green LEDs into the gauge cluster of my 67 GMC to make the gauges easier to see at night. This worked so well for me that I went and bought more green LEDs to install them into my 68 C10.
I always feel a sense of apprehension whenever I am endeavoring to alter something on one of my trucks that is already in working condition, because it seems that this is when the "gremlins" strike! How does that old military saying go? "If it is not broken do not fix it". I felt no such apprehension doing this to the 67 because I actually used parts I bought from board members to assemble the gauge cluster, so it was not working until I completed the assembly.
But tonight I removed the far left gauge-illumination bulbs from my 68, installed the green LEDs, and they glowed perfectly. But when I got to the right side ones (either side of the fuel gauge) and installed the new ones, NEITHER would illuminate! I removed them several times, bent the connectors upwards, switched the polarity, tried again with filament bulbs, and fiddled with them. NOTHING!!
So for some reason, just because I removed two bulbs from the gauge cluster, now NO bulb will work in those slots but the other side is lit perfectly! Jesus Christ, I HATE electrical gremlins!!!