The felt has been know to gum up, depending on fuel/oil combination quality.  You can either fit a spare felt, or remove the felt and knead it in clean fuel to get rid of all crud and gum. When using a glass bowl for that, be surprised of the residue that is washed out, despite using a felt in your fill bottle.
After washing and a good rinse after fitting back, the clunk is ready for another year or two.