Last year I tiled a shower in a rental apt. It's about 3x8 with the shower itself taking up about half the area. Floor was concrete and I was tired of painting it. I used small octagonal b/w tile. The tile outside the shower is holding up fine but the tile in the shower itself is failing. Some have popped up and others can be moved around in place. What might be the cause of this? I followed directions on the setting compound and the grout. I'd like to avoid this problem in the future. I assume the volume of water hitting the tile is the cause of the problem since the area outside the shower curtain is fine. I've seen a lot of tiled showers that seem to be holding up fine. What did I do wrong?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13