I suggest to use adhesive on the wall and back off the tile as well.

If the tiles sounds hollow it means there is not enough adhesive. They may break or comes off
Tiler should use right thickness of adhesive bed depending of how bad was the surface where he lay the tiles.
On poor quality surface it's good to spread adhesive on back of the tiles and floor to be sure that bond will be possible the best.

most of the issues will be down to prep and use of the wrong adhesive also if he hasn’t used a solid bed in the back of the tile also it will break away! try to find out if the wall itself can hold the weight per sqm also! depending on when it was tiled that may have issues that come along with that ie if it was to cold or damp before the adhesive had time to dry also if it was to hot and went off to fast

Yes this is most likely due to prep and needing the right adesive if it's sounding hollow there is not enough adesive and would suggest putting adesive on the tile as well as the wall so you get the right applicable adesive to the tile this will stop it sounding hollow and will help it to adere to the surface you are tiling