Ideally the tiles need to be taken of and re done! eventually moisture will build up and pop them off!
Never, just "dot and dab" tiles, they need a full bed of tile adhesive for them to be secure, and I mean the cement adhesive that has to be mixed , NOT ready mixed tub adhesive!
Not what you wanted to here, but I've been behind tradesmen rectifying jobs just like your explaining.
All the best