i bought a .223 from the gun show a couple of weeks ago, it was doing the exact same thing. didn't notice it when i bought it. here's what most likely happened to your gun. When it was converted, someone probably used an ejector meant for a rimless cartridge. But, the rimless cartridge barrels have one more part in them. It is called the "Lift Button". Someone had left this button and the "Ejector Catch Spring" out of my rifle allowing the ejector to just hang down just enough to not catch the rim. I would be willing to bet that's your problem. It would be easy for some to leave that part out of your gun, considering it probably never had it. I believe on the rimless cartridges have it. Because when you put the case in the chamber it has to push down out of the way. But spring back up into the groove to be able to eject it. you can get this part from
www.brownells.com the part# is 427-400-461 and only cost $4.58
i hope this helps, pm me if you have any further questions about it. i liked to have never figured it out, so i'll be glad to save you some pain. Merry CHRISTmas