Agree with everything that's been said. This should be an easy fix hopefully. I'd go so far as to detail strip the trigger group housing (it's not as hard as it seems!) to look for crud or burrs that may be interfering with the bolt hold-open.
This tutorial page
shows you how to install an aftermarket auto-release bolt plate, but in so doing, it will show you how to detail strip down to the factory bolt release plate.