I have run into quite a few of those. Make sure that the locking screw on the right side of the chassis is completely out and then remove the barrel whip screw completely from the chassis. Clean it up with a tap as some times there is a burr that causes binding. If you don't have a tap, lube the screw good and work it in and out and this will sometimes clear the burr out. If the screw is already locked up tight and stripped out your only alternative will be to return the stock to Sage for replacement.