[Sven]

Been reloading a bunch of .40S&W and having trouble with the Dillon decap pin not punching out all the primers. It happens maybe once every 5 to 10 rounds. Brass is mostly Federal or Speer nickel and happens to both. If I remove the brass, index my 550B forward and re-insert the brass, I can cycle the reloader two to three times before the primer gets punched out. Pin is not bent, die is adjusted to make contact with shell plate and all is clean. Is it a weak spring on the decapper?

 

[RickB]

I think it's more likely a burr or rough spot on the decapping pin itself.
I've had intermittent problems with the issue, and a little polishing of the pin tip with fine abrasive takes care of it.

 

[BayouSlide]

In addition to polishing, put a small chamfer on the end of the pin. Otherwise the primer may be sticking to the pin enough to reinsert the spent primer on the upstroke.

 

[Jiimk60]

Can you flip the primer out with your finger? If you can then it was pulled back in.

I have seen this as a common occurrence with needle cleaned brass that was not completely dried. A little corrosion forms between the primer and the primer pocket.
When the pin contacts the primer, it stretches out before the primer moves in the pocket.
Check your primer, is it still flat with the firing pin indentation, or is it now dome shaped.

Also check the brass and see if it is crimped military style. Yes Virginia there is 40 S&W crimped brass. Crimped brass usually has a 2 digit date of mfg. i.e. 14 or 10 to indicate 2014 or 2010. All labeled WIN NT is crimped.

 

[MifflinKid]

put a small chamfer on the end of the pin

Do this. You can also polish the tip using some Mothers Mag and Wheel polish, Simichrome or Flitz.

As best as I can remember, I have only had this problem on my Dillon dies with small primers