It's probably clogged with years of soap scum and related crud (I'll leave it to your imagination). A few soaks in screamingly hot water with a bit of dishwashing liquid (*not* automatic dishwasher detergent) with exercising the adjuster a few times in between soaks should probably loosen it up. Avoid the temptation to use light oil or other lubricants - they're dirt magnets.
Oh, and while you're operating the adjuster, don't have the razor in a fully closed position back it off a quarter-turn, or so.