Most likely the ejector mechanism is gummed up. There is a disc visible between the hinge ears. There should be a hook on that center disc that engages the edge of the frame when you open the action. It is that disc that catched the end of the ejector rod and forces it up. The rod/ejector is spring loaded and when the disc reaches a certain point, the notch inside clears the ejector rod and allows it to snap back into place. Try ungumming the ejector mechanism and shoot some stuff down the "quill" - the tube that the ejector rod fits into. If it still is intermittent, check to see that the hook on the disc is not worn down to the point where it doesn't contact the edge of the frame just in front of the trigger guard.
