Failures-to-extract are generally caused by one or more conditions, such as a dirty chamber, ammunition issues (under or over-powered, sizing issues, etc), weakened recoil spring, damaged/dirty extractor, weakened extractor spring, shooter's grip, just to list some.
If it's an older gun, I'd replace the recoil & mag springs first thing. Who knows when they were last replaced? S&W recommends replacing them in duty guns at either 5 years of service, or every 5K rounds fired.
I'd stick to using ammunition made by one or another of the major American ammo companies, and not necessarily from their lowest cost/budget lines.
Budget ammo often costs less for a reason. Ammo loaded right at the threshold of the lowest velocity (or falling below it) can sometimes exacerbate things if other less-than-helpful conditions exist, such as a less experienced shooter's unsupported grip technique (unlocked wrist and less than firm grip) or dirty chamber. Variable dimensional differences might also put things like the case rim thickness out at the very edge of the thickness spec, or an overly long case (mouth not trimmed to proper spec). I've known of both happening with one maker's brand of low-cost/low-bid duty ammo.
Now, a failure-to-eject can also involve a weakened extractor spring, or a damaged ejector.
The older 3rd gen guns had shorter ejectors, and also had a sharply cornered angled underneath the tip (where a stress riser could sometimes occur, and where the tip might snap off). The subsequently revised ejectors had longer tips, for faster ejection (especially with hotter ammo), and the angle underneath the tip was changed to more of a curve (to help reduce the potential for a stress riser).
The slightly curved part of the bottom of the ejector tip is part of the magazine safety system, allowing the left magazine lip to catch under the ejector and lift the ejector (so the gun will be able to be fired with the magazine seated).
Here's a picture of an older and newer 9mm ejector, with the older ejector design on the bottom.
The camming shoulders down inside the frame, where the barrel lugs run when the barrel is dropping down during unlocking (and running back up again when rising to return to battery), can experience some peening in the aluminum frames. I've been told by several folks from the factory that this is generally considered to be normal. All of my alloy 3rd gen guns have it to some degree.
I've seen far too many owners/users fail to lubricate their 3rd gen guns well enough so they run normally. If you can't see and feel the oil on the frame rails when you remove the slide (top of the rails, as well as the high & low outside frame rails surfaces, and some on the forward dustcover inner rails), you're probably running the gun too dry. This can accelerate wear and create the potential for functioning problems.
No, the oil shouldn't be present in excessive amounts so that it runs off under gravity, dripping down the gun.
As an armorer, I've had to correct more than my fair share of "problems" created by excessive and/or improperly placed lubrication. (Firing pin channels and extractor recesses which have been contaminated and fouled by excessive amounts of solvents, oils, CLP's, etc are annoying.)
Releasing the slide to run forward after dropping a round directly into the chamber is a good way to damage the extractor. Loading the chamber is intended to be done via the round being stripped from the magazine (so it slips up under the extractor's hook). Fitting a new extractor requires tools and knowledge of how to file on the extractor's adjustment pad, as well as checking the extractor spring tension ($150 force dial gauge needed) ... which means a trip to a gunsmith familiar with S&W pistols or the factory (or factory authorized service center).
Just some thoughts. I have no way of knowing what's happening with your 6906 because I'm not there to watch you shoot it, or examine the gun & ammo and shoot it myself.
I've helped support a fair number of 6906/5903's, though, and fired upwards of 45K through one of the issued 6906's I used for a few years. They're a pretty decent 9mm pistol when they're maintained well, and used with good quality ammo.
