There are usually 2 types of thin set at most of the big box stores.
One is used on concrete or cement board.
The other has a polymer in it and allows some flex.
Another option could be to use a adhesive, like liquid nail. In my BS house I had problems keeping tiles on my stairs (I will never put tile on stairs again due to expansion and contraction). I finally resorted to Liquid Nail and no tile has popped since.
2002 Monaco Windsor 38 PKD
2005 Jeep Grand Cherokee w/5.7 Hemi