I installed some ceramic tile at the base of a walk in shower. The tile are installed over ceramic tile to act as a finish edge. I used mastic to glue them on. Well they came off! To install them I caulked the top and bottom edge and grouted the vertical seams. Apparently water got behind the outer tiles and softened the mastic. I'm really not sure how except that I must have used the wrong product to attach the tiles. What should I use to attach tile to tile and be water proof?