Examine the grout work around the area where you suspect the leak. Remember that a leak may originate in a distant location and wick or run to the point where you are observing a problem. A break in the grout or missing pieces of grout are almost always a source of leaks, but they may not be the primary leak source so don't just assume you found the leak or you may have to repeat the repair on the real cause of the leak. Look closely at the grout and the sealant rim that goes around the tiles. Even fine hairline cracks can allow water to flow through and seep into the surrounding structure.
Remove all the old sealant and grout work from the tiling. Make sure that you have removed all the affected area. It will be easier to just remove the section of tiles and replace them with new tiles. Clean the surface of the wall or floor where the tiles are to be placed. Faulty installations are often due to improperly cleaned surfaces and moisture on the surface. Once the surface has been cleaned and prepared you are ready to place the new tiling.
Fill the bathtub halfway with water. Most occurrences of failure are caused by small cracks that are created under the weight of water or settling of the building. Filling the tub halfway before placing the tile will allow for the slight movements of the tub when taking baths or showers.
Place the new tiling on the wall with a sealant completely covering the edges of the tile. This will ensure that the tiles are sealed and will prevent water form seeping in to cause damage. Once you have finished replacing the tile and re-grouting the tiles, run a bead of silicone sealant around all the edges of the tile. This will seal any water from seeping in along the edges of the tub or wall.