Very good chance that, with that many rounds through it, you have crud packed behind/under the extractor...which is not something that routine cleaning will handle. This causes the extractor to not get as good a "bite" on cases, which, of course, will not always allow correct positioning of the fired case to strike the ejector correctly and MAY be all or part of your problem.
Could also be several other things...your chamber could need a GOOD cleaning...as in wrap #000 steel wool around a bronze brush and scrub like Hell...could be your brass if these are reloads, it gets chewed up and rough after awhile...could also be you need more taper crimp...could be weak springs (yes, too weak springs can contribute to FTE, as well as springs that are too strong) certainly, the gun has earned a detailed cleaning and new set of springs after 15,000rd. of .40's.
Obviously, it could also need a new part or two.
Finally, remember there are eight steps in the cycle of operation:
Feeding; Chambering; Locking; Firing; Unlocking; Extracting; Ejecting; Cocking
Since you have fired so many trouble free rounds through this pistol, it is probably not you that is the problem. Use the steps above to start your "trouble shooting" process of elimination...and always work on the cheapest part.
Good Luck