The theory I've seen is there are two places that the copper can dope into the lead - one requiring higher energy. Likewise, the higher energy location is required for superconductivity. Right now, it seems to require a bit of luck to both (1) get that higher energy location to sit (2) getting a large enough grouping of it to be meaningful.

It's kind of like playing Skee-ball at a fair. You aim for the center, but often miss into the outer rings. For this to work, you need to land a bunch of throws in the center in a row.