Being that the 6.5 is a fat bottom girl, that is the case has such a big diameter, sometimes if their timing is not just right, they can bounce on the ejection port and get trapped.
On your next range run, try to get some slo-mo with a camera or a phone and you can get a better idea about what is causing the issue.
Greg is right, the extractor/ejector combo is a good usual suspect. But you might also be running into a tight chamber that is knocking the bcg out of time. If the bcg's travel isn't smooth sometimes they will lose their grip on the case and kick the shell out early and the ejector might release it before it clears the ejection port. I would recommend polishing the chamber all the way to the neck as well as polishing the feedramps. I run ASC mags, and I haven't had issues with them causing a stove pipe. That doesn't necessarily mean it isn't, just that I haven't seen it with the rifles that I build.
You might also have a burr on your extractor that might be hanging onto the case too long. I have also seen some extractors that are not quite cut right, a bit too deep, that causes a similar issue.
There is a very fine line in the balance between the extractor and ejector on the 6.5 because of the diameter of the shell.
Good luck.