The firing pin end shape that seems to work best is rectangular, with a width of about .05-.06" and a height of about .09-.10". Some makers have used a ball shaped tip, set to keep the entire strike area inside the case head; others design the firing pin tip to extend outside the edge of the case head, that is across the primer area. Position is probably as important as the shape of the firing pin tip.
Also don't overlook the possibility of excess headspace. If the cartridge is not properly supported, ignition will fail or be erratic.
(All the advice, here and above, assumes the ammunition is reliable and has been tested in another gun.)