A wash with dish soap and a rag should help. Rinse and dry throughly. A most perplexing source of trouble is humidity. If the machine is stored somewhere that is humid, you haven't got a prayer for a good spark until the apparatus is good and dry. Store the machine in a furnace room, or somewhere else that is known to be dryer than normal.
Some folks say a hair dryer can help restore function.
If there is some way for the electric charge to leak off the prime conductor it will not build sufficiently to make a spark. This is as simple as making sure it is mounted on PVC or another excellent insulator. Charge can leak off of sharp edges and points on a conductor as well. If you have read through the website and checked out some of the sources in the bibliography you are way ahead of where I was in the beginning. Producing high voltage from friction is tricky business, there are too many things to list that can hang you up.