I'm definitely no expert but I've had this happen to one of my evods where it won't fire on my vamo, too...
I usually have to tighten it down/screw it in more on the Vamo so it makes more contact with the battery.. It's a little bit iffy, but when I do that, it'll finally fire. It's just that one evod/atty head that does it so it's being temperamental at times. Hope that helps! =)