Hang fires are most common when using ball powders in large cases at lower loading densities. Slow burning ball powders have a heavy deterrent coating which makes ignition more difficult. Quite often, increasing the powder charge (providing the charge is low enough that you can sfaely do so) will eliminate the hangfires. If you want to use lighter loads, there are powders which are more tolerant and will work well at lower pressures and low loading density.
Hangfires can also be indicative of a mechanical problem such as a weak striker spring or dragging sear or striker. GD