Hi Dobsy,
I think the "collision cone" for the fire is shorter than the visual effect, so you have to let the spiders get closer in order to kill them. I am currently updating the game and will be fixing this.
There are 11 different places that a refill can appear, there will always be one can in the level. As soon as you pick it up another one will spawn. A random spawn point is chosen by the game from the 11 possible. My strategy is to look around until I find the can, then hold out at that location until it I'm nearly empty. Refill and go looking for the next one.
Thanks for telling me about the garage door. I didn't notice that! I will add that to my list of things to fix.
And thanks for playing the game! Do you mind if I ask what headset your playing on and what you movement preferences are?