Recently fitted XO shifters and while I had the chain off I gave the existing Shimano LX front der a clean and checked the adjustment. I noticed that minor adjustments to the low screw cause definite and noticeable movements of the cage but the high screw seems totally inert. I could screw it all the way in and one heck of a way out, but the cage just does not seem to move. Yet, the chain holds fine and there's no slip.

Does this sound like the high screw is a dead 'un? Maybe there sumtink broke?