I would not completely rule ammunition as a possible cause. Primers not fully seated in the pocket very often will not detonate on the first attempt. They move forward in the pocket thereby negating much of the energy imparted by the striker. Second attempts at firing the same round usually are successful.

I see it mostly with reloaders who are not diligent with primer seating. However I have seen numerous times with factory ammo in the last few years. I have attributed it poor quality control.

You may have identified the problem with the spring you recently replaced but keep my thought in mind if you see the problem again.