Hello The farmhouse kitchen walls are tiled with 6"x6" ceramic tiles. I would estimate this was done approx. 30 years ago. The wall substrate is sand and cement. One wall bounds the living area which has an aga - this is installed on the opposite side of the kitchen wall. This wall seems to have greater problems than the others. The problems are that the tiles have started to lift and the substrate behind looks quite porous. Some have been refixed using a two part instant adhesive. What would be the likely remedy to fixing new tiles ?