My soldering has gotten much better but I still have an ongoing problem. On my Weller 40 watt and Radio Shack 20 watt pencil iron the tips just go dead. It seems like I am always replacing them. I always immediately clean them off with a wet sponge after each use, even between events during one soldering session. Yet all of a sudden they won't melt butter. What is going on here and is it just a fact of life they have a short shelf life?