You can clean a fog machine with a mix of 50% clear vinegar and 50% distilled water - Empty the remaining fog fluid from your fogger tank and put that cleaning mix in, then turn it on and let it run (fog) in a well ventalated area (preferrably outside) until the blockage is gone and you're getting good throughput. Then unplug it, drain the remaining cleaning solution from the tank. Put fresh, good quality fog fluid in and you should be good to go. I'm told that "good" fog fluid will never clog a machine, but cheap fluid will. Cheers!
I recently cleaned the nozzles on 3 of my machines that were 11, 10, and 8 years old. I carefully twisted the nozzles off and then soaked them in a 50/50 vinegar distilled water solution. After soaking, I cleaned them off with a dry cloth and cleared out the holes with a thin wire. Yes, I could tell a difference when holding up the nozzle to a light source. The holes were noticeably larger and unobstructed.