Fekin

All I can say is what everyones already said, dot and dabbing is not the propper method for any form of tiling, never mind Travertine.
All tiling needs to have adhesive covering the whole rear surface area of the tile, period.

Stand your ground and insist they fix your tiles correctly, and redo what they have botched already, and dont pay a penny until they have fixed their "errors"

And no adhesive company in their right mind will say their product will preform propperly with just dabs of adhesive.

Basically if they are only putting a dab in each corner of the tile then thats the only aread holding the weight of the tile, so if they adhesive covers the whole back of the tile then the weight is shared out over a greater area, meaning they tiles will "should" never fall away from the wall.

If their doing it on floor tiles then within weeks they could all start cracking, maybe sooner.