Ox--I decided to try a variation on your roots-in-the-water experiment for three of my rooted cuttings that had stopped growing and seemed stuck with healthy-looking but tightly-closed terminal buds. I wondered if they just needed a more constant water supply, so I put their soda-bottle pots in a very shallow (less than half an inch) tray of water, which usually dries up at some point during the day. Within a few days the buds had all swelled up and two have started to open.
About the same time, I increased the water to several other cuttings I had transplanted into 1-gallon milk jugs. They had also stopped growing, but most are now showing similar progress, and some have already put out tiny new leaves. I'm still hesitant to keep their roots wet all of the time, but so far it's been a pretty rapid improvement. Thanks for the idea!