Sorry for the slow reply, been overseas and cannot see the site from there.
Typically you use a thin set for setting the tiles in place, but you may have another issue if the tiles are popping off it could be they are under some kind off stress from movement or compression. If they are not under stress of some type go to HD and get the thin set used for shower / bathroom application that is waterproof to set the tiles. Then use waterproof grout for around the tile after the thin set dries. you probably need to drop the waterline and blend in the new grout to make sure it is not apparent where you replaced the tile. I have done this and re grouted a complete side in order to hide the new tiles with the old, it only took a few hours to do the whole project and looked perfect when done.
Thanks
Derek