I have one of these things and like it quite a bit with the plastic stock.
I am also very happy with the accuracy with a prb and a maxi.
If you have never before used a flint lock that itself could be the problem.
At this time my best shots before failure if there is such a term is 62 that is in multiple sessions I shot the thing 62 tiems with out a misfire. Then I started to not pay attention and oops.
These things are not that hard to shoot but that does not mean you dont need to know anything.
There are a lot of things that can go wrong dirt, not clearing the fire channel before each shot, flint placement, flint to loose in the hammer, flint to far foward, flint to far back, flint not straight against the frizzen. Touhing the flint edge with your finger and gettin body oil on it, etc, etc,etc.
If you got the thing used you probably do not have the original owners manual.
TC makes two that you could use that talk about shooting flint. Their original Traditional owners manual and the one that came with your Firestorm. You can get both from them at their online sight and it has pictures as well.
A lot of good knowledge there.