[Blankwaffe]

Could be several things.
First I'd check and see if the chamber is fouled badly.
Could be your load specs,brass and or powder too.If your loading too light the cases may not be expanding enough to seal with the chamber walls allowing excessive fouling to build up in the chamber.Ive had that happen before.
If that one round was by chance loaded too hot or too long it may have caused excessive pressure and the case to stick.Tired brass can do it too.
Whats the primer look like....any signs of pressure etc.?
Is the spent cases excessively blacked on the exterior?
If so how far down?
I'd personally clean the chamber and bore real well,then get some different ammo and see if it does it again.