BudsGunShop.com - Light strikes

ammo can also cause light strikes by having harder than normal primers or primers that are not 100% seated. Out of spec brass (rim size can also cause light strikes as the cases don't headspace correctly. the gun can have a bad firing pin or spring which will cause light strikes. if you have light strikes with quality ammo then I would expect (and know) that S&W will repair the gun

If you have had lighter springs put in, you can get light primer strikes. You need to get a firing pin with a longer travel to compensate for this. That will solve the problem in any case.