I am having a problem with laying ceramic tile in my basement. I am laying down directly to the concrete flooe. It is only a 3'x4' area at the door entry. I used some old mortar that I had and set the tile. After a few days about 5 tiles had a hollow sound when I tapped on them so I pulled them up and reinastalled them with new mortar. I am still having the same problem. Also seems like the mortar is not drying after days when I pulled up the tiles it was not solid mortar.