Maybe the back “edge” (90 degree corner) wasn’t sharp enough? What works really well is a piece of hacksaw blade. Put it in a vise or use pliers or something and bend it back and forth until it snaps to a couple inch length. Then use sandpaper to get rid of any really sharp edges. Another tip, what works really well for the magnesium is to use the metal file on a multitool to make shavings. I never use the mag bar though myself, just the spark rod. (Along with cotton balls and fatwood, sometimes just cotton balls, and sometimes not even that)