It has been installed on a concrete slab for about five months now, could the slab not have cured right or is the tile layed wrong?
the tile layed wrong or the slap is moving take one of the cracked tiles up and clean under to see if there is a crack in the slab if there is you need a membrane to put over the crack and then the new tiles good luck
probably there was dirt or something under the tile proir the installation. or the adhesive was not put on properly. you will need to remove the tile, clean the floor completely and re install, and re grout.
It is common on a poorly made one such as yours. People who laid them for has no masters degree, thats why they make a living of laying tiles. The work should have guarantee for one year and Im sure you kept some extra tile. Now you can get them to fix it for you with no charge.
here's the deal about laying tile over a concrete slab. either the tiler has to essentially build a floor over the slab (slab, then firring strips, plywood subfloor, thinset, backerboard, thinset, tile), or must use what's called an 'isolation membrane' to install the tiles directly on the slab (slab, then membrane, thinset, tile). the isolation membrane is a self-levelling rubbery substance that thinset can adhere to, that allows the slab to crack underneath without the cracks propagating upwards through the thinset and cracking the tile. if the tiler did neither, it's almost a guarantee that eventually the tiles over the slab will crack. (yes, if the slab had not fully cured, that might have caused the crack underneath; however, most all slabs will eventually crack even if everything else is perfect. that's the reason for the isolation membrane.)
