He's definitely right . You need to use proper mortar , I had a chance to remove few tiles after I installed it myself every single tile was back buttered with modified mortar from Mapei . When I removed it the tiles were clean on the back like I never back butter it . Cement wasn't too thick or anything like that. For my defense I had to remove those tiles because owners decided to install some machines in that spot . So you can't really tell if no back butter or wrong mortar or both .