The belt tighteners on these engines aren't very durable. There is probably internal slippage in the tightener assembly which is preventing the idler from being moved far enough to tighten the belt. Once you loosen the 19mm nut the tension is lost, and that's it for the tensioner. Now that it slipped, the tensioner nut can be turned until the tensioner rod contacts either an object, such as a bolt, or the rod will start contacting the side of the tensioner itself, usually breaking the tensioner cam, but occasionally the rod strips out, as yours may have. You need a new tensioner is the short of it, and now you probably need the tensioner tightening cam and rod, and I usually recommend the tensioner shock absorber as well. I usually make a cursory inspection of the belt tightener before removing the serp belt in this manner: If the threaded rod is very close to the bottom of the tightener nut, I make sure the service advisor knows that the tightener is suspect of being worn out; the farther down in the nut the rod is, the more the previous mechanic had to turn it to get the belt to proper tension.