Some Cetme / HK rifles have the ejector problem TACT indicated.  Sometimes it is the trigger box, sometimes the reciever itself.  Either way, I have had good luck adding a little weld on the trigger box (NOT the grip frame!) then carefully filing on the weld until the trgigger box will JUST fit under the ledge and go against the reciever in the back.  The grip frame is just along for the ride and is not the locating element here.

HK & Cetme are delayed blowback guns.  They use a "fluted chamber" to help the spent case let go of the chamber.  If those flutes fill up with crap such as the TAR sealant of Cavim, they will have all kinds of extraction / ejection / feed problems!  Clean well, clean again then try again!!  

Check the headspace too.  Easy - remove trigger pack, put stock back on.  With no round in the chamber, let bolt SLAM closed.  Measure distance between bolt head and carrier.  .007 to .021 clearance I think.  Will look it up if you need me to.  Two ways to correct.  Either reset barrel depth, PIA, or go to larger rollers.  Oddly enough, headspace is not real critical in these but will act REAL goofy if bad enough.

Note:  Make SURE the bolt carrier's cocking "snout" does not rest against the cocking piece!!  .002 to .015 clearance required or headspace is impossible to measure & set and gun can be VERY hard to unlock / cock.  Slather carrier with break free when gun is new.  More reasonable amount will work after worn in a little.  "Snout" must not have undue drag on cocking tube from burrs or misalignment.

I have seen some Century guns that mag did not go in properly.  Make sure it locks in OK.  Try another for good measure.  They seem to leave out part of the catch assemby making the spring get out of place.  Ejection lever must be free in trigger box.  Often the trigger box is damaged a little and the ejector "hangs".  It is spring loaded and the spring is tiny so too much drag and it is late moving up to contact base of spent case causing failure to eject.

Low powered ammo is a PIA in HK!  Will extract fine but short stroke and jam.  Speaking of short stroke - anything that causes recoil spring to "hang" will cause problems.  I have seen Century guns missing the little metal / plastic ring at front of recoil spring.  Even saw a new one with broken recoil spring!  If you need to replace ring, do not bother to re-rivet the part in end of tube.  Buy two small roll pins (tension pins) at the hardware.  Drive in and file or grind flush.  Works great!

Hope this unorganized ramble helps!!