We have about 400 to 500 sq. ft. of tiled floor that was laid down about 15 years ago. We now have 5 or 6 spots where the grout has come out. These areas range from 3 to 4 inches in length to a foot or so. In one area, the missing grout looks like a cross. I have repaired the grout twice and each time it came up when vacuumed about a week later. What is the trick? I have been trying to phone you during your Saturday show for the past several weeks now and cannot get through. As I have the house up for sale now I would like to repair this blemish as soon as possible. Any help that you can give will be greatly appreciated.
Your sub-floor (sub-straight) may be moving causing the grout to come out. My suggestion would be to remove the grout and replace it with epoxy grout. It costs a little more and is not as user friendly but you have very little choice with this problem. Ask for the epoxy grout at your local tile supply source.