I noticed my fogger wasn't working as well as it did last year. As a last resort I read the directions. The nozzle of the fogger unscrews and builds up residue restricting the amount of oil to make fog. With the nozzle off I pumped it until clean oil came out of the tube. After cleaning the nozzle and pumping out the gunked up oil in the tube the fogger works like brand new.
It's also important to let the fogger burn for a few minutes after your last pumping to give it a chance to purge the last of the oil.