Quoted from Mk1Mod0:
In a word, connectors. Do a continuity check from the bulb socket to the SCR. That will give you the answer. Dirty or corroded connector, corroded plug, broken solder joint on the pins...etc.
Solved my problem on my #Eight Ball. The chart I was using (and the schematic too) have the positions of two of the lamps swapped - the credit lamp on the apron and the "Same Player Shoots Again Lamp". They are shown on the schematic as Q60 -credit and Q52-SPSA. But I realized by testing them again (by jumping ground to the Anode of the SCRs) that despite the schematic saying that Q52 should be controlling the SPSA lamp, it was lighting the credit lamp in the apron and conversely Q60 was lighting the SPSA lamp. I hadn't noticed before since the credit lamp is off to the side and I had been looking at the main playfield lamps when I was testing things.
So since they are swapped out, I had replaced the wrong SCR. Well, it should have been the right one, but anyway... I replaced Q60, which should be the credit lamp SCA and voila, my problem with the SPSA lamp is fixed...
So either the schematic is wrong, I'm reading everything in the world wrong or my machine is just wacky, but all is well with the world because both lamps light at the right times.
Maybe someone else will have the same problem and see this post later on.