Nothing on the pistol should wear out in 400 rounds unless your extractor spring was marginal from the start, incorrectly tempered or something like that. I don't think that is a common occurrence but anything is possible. I would call KT (service, not parts) but I doubt they can diagnose it over the phone. There are too many variables and they might ask that you send it in. Short of that, I would start with them sending a new extractor, spring and screw. You may as well ask for new recoil springs too. I would also check everywhere on the gun (slide rails, hammer interface, barrel lugs, etc) for any undue friction slowing the slide. Maybe there is something here that you overlooked.
http://www.gunsmith.fuselier.com/function.html
http://www.1bad69.com/keltec/fteftf.htm
http://www.gunsmith.fuselier.com/function.html
http://www.1bad69.com/keltec/fteftf.htm