If you are shooting 115gr 9mm, try going to 124gr or 147gr. When I was at Glock last month, someone brought in a 17L with a similar issue and the tech recommended against shooting 115gr loads. YMMV
I called Keltec customer service and they sent out a new extractor and extractor spring. Of course, at the time I owned 2 PF9s and had swapped extractors and had the problem move with the bad extractor...