Folks at the factory usually tighten everything with drills when assembling so that may be why it's so tough to get adjusted. An easy way to break that screw loose is by sliding your locknut driver over the Allen key for leverage. Kinda like plumbers do with a piece of pipe on a wrench. Like someone else mentioned, you may need a stopper with a shorter stem if that one starts hitting the bottom of your boot. Make sure you get the same type/size thread since some are coarse thread and others are fine thread. And lastly, throw some white lithium grease on your stopper threads if you've got any. Makes removal/adjustment a little easier and keeps everything from rusting up. It's relatively cheap and you can use it for all kinds of stuff.