Edarnold
Member
There is a projection on the tail of the extractor, where it slides into the bolt body, that engages a slot on the bolt head. This holds the bolt head extended against the force of the firing pin spring. Normal wear from use rounds off the projection, until it becomes unable to keep the bolt head extended when the bolt is removed from the receiver. The fix is a new extractor, if you can find one, or some careful work with a file to square up the profile of the projection.