Sometimes, for old probe, its not chemical, but its the physical layer that deteriorate the conductivity of the surface.
Like the Fluke ones, I guess as it aged, usually finger fat, grease etc you name it, formed at very thin layer at the tip and will affect the surface conductivity.
Usually I dip the tip only into acetone (beware, not allowed the liquid to touch the plastic part), and give the tip a good strong rub using tissue paper or hell, even used news paper, then it will perform as gold ones.
Yes, even gold plated ones, when aged especially used alot, it will be covered with grease/fat/oil, and even the gold doesn't react with them, but still the dirt layer will affect the performance.