Why won't my propane combust when I spark it?

I need to run an experiment and have some pretty basic questions. I'm trying to get combustion of propane in a controlled environment. I understand that that looks like this:


$$\ce{C3H8 + 5O2 -> 3CO2 + 4H2O}$$


Meaning I need 5 times more $\ce{O2}$ molecules than $\ce{C3H8}$. But, I want to do this based on the volumes of the gases at STP. Do I just go with 5 times the volume of $\ce{O2}$ to the volume of $\ce{C3H8}$? Do the gas laws require that I take into account the size of a molecule?


And seeing that $\ce{O2}$ comprises about $\pu{21\%}$ of the volume of air, I would need to multiply the volume of oxygen by roughly 5 to get the correct volume of air needed...yes?


That gets me a 25 to 1 ratio between air and propane by volume. So, I tried it, but I couldn't get it to burn.


Doing further research, I see something called the Limiting Oxygen Concentration, but I think either I don't understand it, or this isn't the problem. 


I also found that the lower and upper limits of flammability for propane are $\pu{2.15\%}$ and $\pu{9.60\%}$ respectively. My concentration is roughly $\pu{4\%}$, so this shouldn't be the issue either. 


The only other thing I can think of is that my spark isn't "good enough". Is there a minimum amount of heat required from my spark to ignite the mixture?


Anyone have any ideas as to what I am missing here? Any help would be much appreciated! 


