All .40 FMJ is flat nose.
First try different ammo, I've never had a problem with American Eagle or Speer Lawman in my G23 and G27. Next I would detail strip and clean the gun, particularly the firing pin channel, extractor, and cut-out in the slide for the extractor. If you still have problems, I'd replace the recoil spring assembly, firing pin spring, and trigger spring with new factory springs.
If the extractor is one of the new "MIM" extractors, there's a small chance it may be out-of-spec and that it's causing the problem. If everything above fails to solve the problem I'd try a replacement .40 LCI extractor.
G23 G27 .40 S&W Club Member# 4520
Stay safe, stay Second Amendment!
Last edited by voyager4520; 10-05-2012 at 11:16..