Carefully check the brass for marks. How exactly does it fail? Does the empty remain in the chamber and the next round feeds into it, smokestack? Also have you check the extractor spring tensioning? The 40 is a higher pressure round and they sometimes get a bit sticky,.,. A mark on the fired case that indicates the extrator pulled over the rim would be a good indicator of a too light extractor spring. First though we need more details.. These are usually faily decent shooters..