OK I would say you are overloading your cumbustion chamber... It has to have some oxygen or air inside as well as your propellant to cumbust. Too much "spray" and you lose your oxygen and it misfires.
Note AXE DEODORANT seems to work best for me on cumbustion style cannons.
Try just a couple 2-3 really quick sprays into it, after that THEN load it, any air in the barrel will to some degree get compressed into the propellant mixture and you should fire ok. If that doesn't do it, check to see if you are getting a good spark. If that doesn't work, get this...
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=170068096487&ssPageName=STRK:MESE:IT&ih=007
Xtreme