You don't mention the caliber of your 336. If it is a .30-30, or .32, these headspace on the shell rim, but if it is a .35 Rem, it headspaces on the shoulder. If factory ammo fires ok, then you are probably setting the shoulder back a bit when you size the cases for your reloads. This is not difficult to do with the small shoulder on a .35.
I don't know if Marlin has made changes in recent years, but in early .336's broken firing pins were far from unknown.