I know that some plugs are just better than others and there must be some percentage of failures out-of-the-box, but I am having a hard time wrapping my head around total inoperability with one type versus another. Not doubting it, because there are plenty of stories to back it up, but I'm fishing for a good reason. You couldn't find a simpler piece of electrical equipment. Really, just two pieces of wire, an insulator and two electrical joints.
So, here are some unsupportable theories in no particular order:
1. Length of thead, either on the plug or in the head, leads to the grounded electrode shading the spark more than exposing it.
2. Technique of fusing grounded electrode to plug body (resistance welding) might create a high-resistance joint. Likewise, cap to insulated electrode.
3. Alloys used might also lead to resistance in joints.
4. Bad batches of ceramic mix for the insulator could lead to unseen hair-line fractures along which spark could travel.
5. Ceramic crystal structure might promote spark travel even when not fractured (talking way outside any technical expertise in this regard - maybe applies to the whole topic).
6. Plating of cap material or plug body could create resistance.
7. Any of the above in combinations
Looking forward to hearing more possibilities or learning why these are ridiculous.
Richard L.
Editing
Thanks Bill, adding to the list:
8. Lack of glazing could lead to the ceramic absorbing liquid and creating a spurious path for spark travel.