Glock 27 gen 3 fails to eject?

I bought this gun brand new and put 200 shots through it, cleaning after every 50 rounds, with no problems at all. I was shooting wolf ammo, which is a dirty bullet but surprisingly everything looked clean. The next 50 shots i used winchester white box ammo and after 20 shots i had my first failure to eject, the end of the casing was still slightly in the chamber. Now using wolf which i shot fine with before it fails to eject about 4 out of every 50. Its not limp wristing because i shot my dads gun 100 times with absolutely no issues, and with mine i really hold firm.I have already cancled out a weak spring by using the one from my dads gun, and we took the extractor apart and it was clean, the gun has maybe a total of 400 shots through it now, any ideas would be greatly appreciated.