Solutions for crumbling/cracking grout on newly laid floor?

[Beewings]

I have recently had new ceramic floor tiles laid by a professional tiler over loose wire underfloor heating on new plyboard over old floor beams in the kitchen of my 1880's house. Despite using flexible powdered grout we are having problems with some of it crumbling, slight shrinkage away from the edge of the tiles or hairline cracks appearing in the grout. The tiler has been back and lifted and relaid some of the tiles as they were moving very slightly. This seems to have fixed some problems but not all. After subsequent reading I assume that there may be movement in the sub floor and that our carpenter should have laid thicker plyboard to absorb any movement.
Does anyone have any solutions to protect the grout from crumbling or deterorating further (eg adhesives or protection stuff) as it isn't too bad yet, but isn't as hardy as I would like. It isn't an option to bin the tiles, the underfloor heating and then fork out for additional labour costs!
All help appreciated - from a girl who has no experience and expected the pro's to do the thinking for her!!

 

[jefoss]

ask him what adhesive he is using, it should be flexible, as should the grout be.

also, if your boarding it needs to be 12mm py screwed in very 20cm min

 

[Mo-S3]

I thought your still suppossed to use adhesive on the tiles to fix them tot he floor even with floor heating. Has he just layed the tiles on the heating wires dry and then grouted? Or am i reading it all wrong?