bought home, tiles loose in shower, wonder if water damage is behind it
take them off and replaster them
Plaster or fresh concrete mix
If water can get behind them, it probably has. I suggest you take the tiles down ASAP. Water in a bathroom can cause MOLD. Depending on your state, if it is moldy back there you can hold the previous owner's responsible. Don't risk your health!
Tiles originally went on with an adhesive mortar and then grout. I suspect the grout fell out, and water softened the mortar and the tile fell off. Take the tile off and clean any loose mortar from behind it. Don't use the shower for a couple of days and let the area dry thoroughly. Buy some silicone adhesive that is not water soluble and re-stick the tile to the wall. Once it has been in place for 20 minutes, re-re-grout the tiles using a latex grout. Let it all dry a couple of more days and you are good to go.
