The hole under the primer (flash channel) NOT THE TINY HOLE ON THE OTHER END (flash hole)......fills up with carbon. Some designs are more prone to it than others. The Lehigh NFPJ conversion plug can go at least 50 shots and still has plenty of volume. I never clean mine until after the range session and never had a problem even with normal Win209s.
You can clean it out with a ProShot SS gas port brush set or a drill bit BY HAND.