I have an older 2-7 Leupold and it doesn't seem to hold zero and sometimes the verticle cross hair won't move with the turret screw. The cross hair wouldn't move so I gently tapped the turret with a small wooden screw driver and then it moved and I zeroed the gun. It appears it didn't hold the zero and now the cross hair won't move again. Any idea what is wrong or what happened to it ? Any ideas if it can be repaired ?