Most pros wouldnt use "adhesive/mastic" on a ceiling in a shower or tub area eventhough it may hold better at first. In wet area it never fully cures and can actually reumusify and then you are back to sqaure one...
You should use thinset mortar...be sure to "backbutter" the tiles so that you get a good suction action when you put them up. You could get a non-sag mortar that may improve adhesion on the ceiling, I have never used it but have been told. This is not "quick and easy" but definetly the right stuff to use in a shower or tub area.
Also, I would ask that you may want to investigate why the tiles are falling off in the first place. Do the old tiles seem to have full coverage of whatever was used to adhere them? What color is the stuff? Does it look concretish or glueish... If for example there is mastic on the back of those tiles, you may expect many more to start popping off..
"What if the hokey pokey really IS what its all about."