If I have 12v+ at the top of the coil, why no spark?

[TIMSPEED]

Huh...this damn truck's ignition stumps me DAILY! Again, it WILL NOT start. It cranks over like a mother bear, and has a FULL 12v+ at the HEI power Wire. Anyone wanna start the guessing? I've got an ENTIRELY brand new Distributor. I've had the coil and module tested MANY times. Last month, it did this same thing, and I replaced the cap/rotor, and it fired right up...again, I bought another cap/rotor and again it'll probably fire right up. Anyone wanna venture a guess WHY it keeps eating cap/rotor's? I absolutely HATE changing them, because they have those stupid L-clamps that NEVER want to sit right...