I am helping lay subway tile in the master shower of my daughter's new home. The subway tile will be on 2 sides, (the other 2 sides are enclosed with glass. The tiled section is 8' ft. high x 6' ft. long and 8' ft. high x 4 ft. long. It started out great- all level, but about 4 ft. high it started getting unlevel by about 1/8th inch. From the front it almost looks like it has a slight sway in the center - its not to noticeable, but I don't understand why there would be a sway at all and why is it unlevel. The wall is square and does not have a bow. Help, I really do not know how to fix this. I know that all tile may vary slightly, but if I tried to put too many wedges, then the grout line will be too wide in spots. What am I doing wrong? (Other than not hiring a professional).