Since the tiles are coming off, you can see what's underneath. I'll bet you have the answer to your question right there. I had recently posted an opinion that, if properly installed, a tiled shower using water-resistant drywall (*greenboard*) should be viable. From the comments I received, however, I have decided that this is an imperfect world, and that simple prudence dictates the use of cement board in all shower applications.
Therefore, the first thing I would do is look and see what's underneath the tile. If it's not some form of cement board, you've probably isolated the problem right there. If it is, then your hunch may be correct that it's related to the *fake* glass block.
Since this is a new home, it's important to know what materials were specified (*spec'd*) for the job. If the materials used were not the ones you paid for, you should be able to make the builder re-do this area at no expense to you. Assuming, however, that cement board was used AND that the *fake* glass block was in the building spec, you would then have recourse to the manufacturer of the plastic block, who may indicate that their product was not installed correctly. In this case, the builder would have to re-do the installation following the manufacturer's specs.