I use brake cleaner to clean the extractor channel. Lube it afterwards.
CRITICAL: n8vmdpath, what tdogg says here is good advice, but it's important to use non-chlorinated brake cleaner. It's not just the threat chlorine poses to your lungs. Chlorine can weaken metal. So non-chlorinated only.
And be aware that it will melt plastic. Be careful what you use it around.
CZs often come packed in Cosmoline, which is super viscous. Even if you have cleaned it, there could still be some gumming up the extractor. I'd recommend you detail strip the whole gun and clean the whole thing with non-chlorinated brake cleaner, then relube (lightly, only at friction points).
Another possibility is that you have an unfortunate combination of variable stacking up just right. I load my own ammo, and I've seen bullets that will suffer occasional failures to feed at one overall cartridge length, but ZERO failures a hundredth of an inch longer or shorter.
It could be break-in. I would expect it to be fully broken in after 800 rounds, but it is possible that you could take those same rounds, put them away for a few months, then pull them back out and find it all works fine.
AND... you could always try replacing the recoil spring
edited to fix seemingly important type-o: breaking vs break-in
