Ok...before the blobby police get here... let me start by stating that although blobbing adhesive rather than full bed 'combed' adhesive is not the best way to
tile, it will not always be the case for failure.
It could be that the adhesive wasn't the correct type, it wasn't the correct mix or that it was allowed to skin over, therefore not having the correct bond to the
tile.
In the last few years, adhesive and
tile manufacturers have been working better together to understand the problems that tilers and customers have.
Blobbing won't necessarily mean failure although it is not regarded as best practice.
The other guys and gals here will tell you exactly what your tiler should do on his revisit.