Dale, this has been a known issue with Glocks in .40 for years. There is a very small part of the chamber right above the feed ramp that iwas unsupported. I am not sure if it is still so in current Glock generations.

Indications would be "smiley" faces on the base of the brass near the rim. REally in my understanding all you have to do is watch the cases and not load cases with smileys too hot.