Maybe extractor is sticking or weak extractor spring maybe, but except for a gunsmith I do not know how to fix that? Maybe try Bob Naess, he fixes Chauchat’s. Link to his web page here:
http://www.blackrivermilitaria.com
or maybe disassemble and clean inside of receiver, I cleaned mine up because of too much drag on barrel and extension with red 3M cleaning pads. Or disassemble and try putting a cartridge into chamber by hand, that is how I found I was using the wrong cartridge. Mine will not accept WWII ammo only the Serbian ammo made in 8mm lebel. Even the hand loads that came with the gun would not chamber unless the bolt slammed it into chamber.