It's call the ejector. It looks like you have fired high pressure loads. Generally in this situation, when the bolt is opened, the edge of the ejector hole will shave brass off of the case head. The shaving get stuck between the hole & ejector pin not letting it pop back up. I'd try soaking it with some kroil or some bore solvent for a few days, then work it up & down to see if it will free up. You may need and ejector spring once you get it out.