There's every chance in the world that I'm using it wrong, but my "simple observation skills" tell me that I can turn the adjuster clockwise for ten minutes and nothing visible will happen. Then I can turn the adjuster anti-clockwise for twenty minutes and still nothing visible will happen. It never comes to a "stop". Not even the amount of metal ferrel (?) showing at both ends of the plastic barrel changes. If I hold the brake lever down, I can't turn the barrel adjuster either way. That's why I thought the mechanism itself might be broken, or that I had previously unscrewed something inside beyond the point of no return. I really don't know how the mechanism works in order to shorten or lengthen the cable. (or have that effect, anyway). I can take it to my LBS or I can buy a replacement and take this adjuster out and dissect it to understand how the mechanism works, but like I said, I'd rather be riding! But I'm still optimistic that I'll get the understanding I need from Bike Forum.
If you have a "stationary" adjuster, turning it doesn't do anything. It doesn't thread into the frame. It uses a threaded ting of some kind to jack it in and out. If you can't get it to work, consider slackening the cable at the pinch bolt -- You don't need to unthread it just pull it back an inch or two -- and pull the adjust uniy out and see if it's a single threaded part or two parts threaded to each other.
It's entirely possible that you're missing one part of a two part adjuster.