Replacing tiles

[davester]

I agree with Michael's and Kraniac's solution. It sounds to me like the problem is a bad installation job and/or use of crappy premixed thinset. You need to chisel up that thinset and then put a new layer of thinset down before replacing the tile. I strongly suggest that you protect the edges of the remaining tile with masking tape because when you chisel the thinset fragments tend to fly out and chip the edges of the remaining tiles (ask me how I know). Also, when you reset the tile, use a straightedge to ensure that the tile is flush to the adjacent tiles. If it's not flush pull it up and add or subtract thinset until it is.