The problem is that the wall is uneven.
Tiling onto uneven walls.
First chip the bumps cracks and dents on the concrete wall with a hammer.
Plastering the walls is the best way to ensure that the walls are nice and even.
How to prepare an uneven wall or floor for tiling plastering.
You want the tile adhesive to have something to grab onto.
Level those uneven walls.
Use a flat scrap of wood to even and smoothen out the surface and let this dry for a few hours or overnight.
Then flush and fill the areas with cement.
Things you will need measuring tape tile wet saw leveling spacers and wedges trowel thinset mortar rubber mallet grout grout float grout sponge.
Thoroughly vacuum the area and give the wall a once over with a damp sponge or cleaning cloth.
Seal the walls with a skim coat and let it dry thoroughly.
Once the cement is dry apply the bonding agent that should give the surface a rough texture.
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.
If you put a 48 straight edge up against the most sunken areas there s a gap of almost a 1 2.
There is also a spot near the window trim where the wall appears to be built up too much enough so that the tile could end up proud of the trim which i m pretty sure won t look so hot.
For an uneven concrete floor a floor leveller will solve all your problems.