Big_D,

Thanks for responding.

I don't think that there is any stress (the tiles that loose are at the top edge of the spa area). We tried a couple of different sanded grouts before (from either HD or Lowes) but we used the same grout under (between the tile and the spa wall) the tiles, thinking the grout itself would be enough, so I guess that we need to use thinset instead, plus the grout between the tiles?

The sanded grout we have is called Laticrete Polyblend, I think.

If you know specifically which brand, etc. we can use for the "waterproof thinset" (the HD/Lowes people in this area don't seem to be very helpful), I'd really appreciate it if you'd post that info.

Thanks,
Jim