Good afternoon, I have two observations to make. First, you should take into account the squareness between two walls, meaning that they should be complete on two sides, not just on one side. Second, you should not step on the tiles that are already in place because your weight will cause them to fall and you will end up with gaps.I set tile everyday. Stepping and standing on tiles that have just been laid is a good way to have tiles not bond properly to the substrate.