I had the same problem with my Spanish 43 and the Yugo ammo. I was used to shooting Romanian ammo which have pretty soft primer with no problems. The 1950's Yugo has hard primers. I got a new firing pin spring from Wolff Springs and now every shot goes bang with the Yugo ammo. If you don't want your Yugo stuff I'll take it.
Try a new spring you'll be happier. JP