The problem is that the wall is uneven.
Tiling with unlevel walls.
Plastering backer boards and self levelling compound are all great options.
There are numerous ways to make a bumpy uneven or unlevel surface ready for tiling.
Back buttering is a trick used for tiles of uneven thickness like ungauged slate or handmade tiles extremely large tiles or for tiles being installed on an uneven surface.
Back buttering is a common solution to tiling uneven walls and floors or using tiles of varying thicknesses.
Short answer to how to prepare an uneven wall or floor for tiling.
Things you will need measuring tape tile wet saw leveling spacers and wedges trowel thinset mortar rubber mallet grout grout float grout sponge.
The area of the wall to be tiled minus the area of the window is 37 square feet.
Believe it or not there are more uneven walls and floors than there are even ones.
Extra mortar is applied to areas that need it either areas with dips in or areas with thinner tiles.
Here is an easy way to set your 1st row and the rest going up your wall then lastly the first row is set.
The mortar is spread.