Although I've never measured the diameter of the wells, I would be (really) surprised if they were not identical. Did you find a large amount of oil on the bottom of the socket? If so, then maybe with enough oil in the wells (comes from the leaking valve cover gasket), the socket is being blocked from going down far enough. The only other thing I can think of is that some of the wires broke off, but I'm sure you would have noticed that. If the wells have lots of oil in them, sop it up with a rag pushed around on the end of a thin screwdriver.