Why do potato guns fire inconsistently?

My question is why does my potato gun, powered by hair spray and a potato for a projectile, sometimes fire with a bang and go far and other times create more of a poof noise and may go only 50 feet or so? I try my best to keep the ammo and hair spray about the same every time but the results are very inconsistent. My theory is that the hairspray may mix unevenly inside the gun causing the hairspray to ignite slower or faster. I don't know if my theory is right or not. PLZ HELP!!!