Why Are My Tiles Not Sticking?

Sounds like the adhesive has a skin on it once it's been troweled, as it's only adhered to the wall or maybe you're not applying enough pressure onto the tile when setting. If you touch the adhesive with your finger and it doesn't stick to it then remove the adhesive and re trowel using a fresh coat.

Why is my tile not sticking to the wall?

If the wall is not leveled, the adhesive might lose its grip. If your adhesive layer is too thick, it might also fail to hold the weight of tiles regardless if you apply it to concrete, cement board, plywood, or drywall.

How do you fix a tile that didn't stick?

If your ceramic floor tiles aren't sticking, it could be due to improperly mixed grout/thinset or not using enough grout/thinset. You could also be attaching the tiles to the wrong base, letting the thinset become too dry before applying the tiles, etc.