I opened the chest to reveal the spark mechanism to see what is the problem
but it's not needed.

All you need is a tiny screw (mine came from an Animated Blurr's head) that will fit in the hole where the spark stone sits.

Don't wind the screw too tight; it will jam the mechanism!

Try to slowly adjust the screw to fit the needed spark.

hope it helps somebody! My Doublecross' shots fire again!