Gen4 Why So Troubled?
It's not just Gen4. Gen3's have the same problem, though it doesn't seem as bad in Gen3 for whatever reason.
My 2004 Gen3 G23 ejects fine, my 2009 Gen3 G27 does not, using the same extractor and spent casing in both slides the G27 has about 1mm of extra space for the rim of the casing to slide rear-to-muzzle between the breech face and claw of the extractor. That's the problem, the extractor cut-out in the slide is too far forward and it's allowing the casing to drop down toward the magazine instead of being held to the breech face.
It seems the problem exists to varying degrees from one slide to another, it's likely a production/quality control issue. Because the extractor claw is still forward of the rim ejection still occurs, though not as reliably. The casing pivots right and smashes the case mouth into the right side of the ejection port, then bounces around in the ejection port.
Glock has a new 9mm ejector to help remedy the problem. Hopefully they'll have a new .40/.357SIG version soon. It's very new and some of the Glock techs have said it's only for Gen4, but even so the ejector would work the same in a Gen3 or earlier. The new ejector has a rearward tilt that would raise the case mouth earlier to help the casing clear the ejection port, and it's taller top to bottom so no matter how far down the casing falls the ejector will still act on it in the same fashion. Every person I've heard from who's tried the new ejector said it fixed the problem. Even if it doesn't address the cause, it's still a solution. But Glock needs to assure in the future that when the slides are cut, the extractor cut-out is precisely where it should be.
Here are some pictures of the new ejector:
1. Ejector Fix pictures by SGTDuffman - Photobucket
Apparently some people have gone to GSSF matches with their Gen4 9mm's and their guns didn't have any problems, but the Armorer still replaced their ejector with the new version. So apparently Glock is standardizing Gen4 9mm's on this ejector. It should only be a matter of time before the large retailers receive the newest Gen4 9mm trigger housings with the new ejector, and if you own a Gen3 that has this problem it's a simple matter to pull the ejectors out of the trigger housings and swap them so you can use the new ejector in a Gen3 or earlier.