I don't wish to be accused of scaremongering or tell you your wetroom is doomed to failure as that may not be the case but from the information you have given the job has been done incorrectly with regard to technique and materials.
Tiles over 300x300mm are classed as large format and ready mixed adhesive is not suitable for the reason that it dries by dispersion - I've removed big tiles that have been fixed with ready mix and the center has still not dried after 2 years.
Even if your tiles were smaller most tilers would use a bagged adhesive in a wet environment.
The adhesive should be combed on to the wall with an appropriately sized trowel as well as buttering the backs of large format tiles to achieve the 100% coverage that is needed in a wet use environment.
The above is just going on what you have mentioned, and would like to also ask:
What size and thickness is the tile?
What type of tile is it? (ceramic, porcelain, natural stone)
What are the walls that were tiled made of?
What kind of waterproofing (tanking) system was used?