Porcelain Tiles/ Underfloor heating issues

[wazzer]

I have undefloor heating in the 2 1/2 year old kitchen and office extension using polypipe, 70mm screed on top, and have 45 x 91cm porcelain tiles in the kitchen. Everything works fine, but a year after installation, some of the grout in the kitchen was cracking between some of the tiles across various areas. We got the original tiler in who, after lifting a couple of tiles, suggested the adhesive had gone off too quickly, but he lifted a further couple and blamed the sub floor after seeing a micro thin hair line crack in the screed, and wanted to charge me for his work. He relaid four tiles which, six months later are still perfect. The others, he just repaired the grout which has all now cracked again. I can tap the centre of a number of tiles and they sound hollow, others are not. Does this suggest he hasn't used enough adhesive/ has it gone off too early/ he swears blind he used flexible adhesive at the start but couldn't remember being told there was underfloor heating when he instaled them). Also, a strange occurrence that has happened half a dozen times over the last year or so, is that we've heard a crack/ pop noise under a couple of tiles and we've felt the area directly around the area vibrate slightly if you are standing on those two tiles so this is a localised thing. Nothing runs under these tiles nor does anything with the extension look untoward so I figure its expansion/ contraction under these tiles. Anyone got any ideas or heard of this happening. Interestingly, we turned the underfloor heating off the day before yesterday and yesterday the crack/ popping noise happened again for the first time in months. The heating etc all works fine so is the floor cooling down? Does screed expand/ contract at all or could there be air pockets under the tiles where they aren't adhered properly that could expand/ contract to exhibit this? Any help with sorting the tiles above or the strange occassional crack/ pop would be most welcome.