Why is my potato gun not working?

I've made a potato gun, or at least attempted to. For some odd reason, I can't get it to fire the potato. I hooked up the igniter wires to where they're able to create an arc in my 13" segment of 3" pvc pipe. I use a cheap aerosol hairspray as the fuel, and I've put a potato in my 5' segment of 1.5" pipe in a way where it's acting as a plug, not letting air through so it can be pushed after the combustion. It seems that it's not lighting the aerosol after I put the cap on, but after I take the cap off and press to ignite, flames shoot out. If anyone can figure out what I'm doing wrong or what I'm missing, I'd appreciate it. :) Thanks