[Serum]

problem is, that the wires are isolated individually, and it are a lot of thin wires used to make one thick one. so if you sand off the outer layer, only a small amount of wires will make contact. Check if you can peel the wires from eachothter, if this is the case, then the isolator is still on and the wire wasn't stripped.

they use some kind of accid to remove the isolater, perhaps this process didn't took place on that particular wire.