One of my tires has been leaking for a while, and it is driving me nuts. I checked the whole thing visually and cant find a single spot where the air may be escaping. I thought it may be the valve stem, but I tightened it, the tire still leaks. There was a whole in it before, but I plugged it up.

Any advice for experienced tire pluggers? It a 245/70/16 PZero.