Grout Cracking despite rakeing it out and doing it again

[Weebisto]

hi all, did a bathroom and a kitchen floor this week. Used standard set addy which gave me a bit more time to work. Left it for a good 24 hours before grouting Plywood subfloor installed by joiner and sealed with latex sealant by me before being left to dry and then tiled. Grout in bathroom fine but grout in kitchen showing lots of cracks. Went back in today and mixed grout a little thicker in case that was the issue, raked out affected areas, cleaned and re-grouted. Been back to check and although not fully dry it's not as bad but still cracking has appeared. Heating on full bung so wondered if it's dried too fast? Alternatively, could it be movement on the sub floor?
Any suggestions appreciated

 

[Andy Allen]

Are the tiles loose..?

 

[Weebisto]

That was my first thought but they all seem sound

 

[Andy Allen]

What adhesive and grout did you use?

 

[Weebisto]

Larsen std set and mapei grout

 

[Rookery]

Almost certainly movement due to poor substrate prep. Or it could be wrong grout. Or it could be a few other things. We need the details of what tiles, how they were fixed, what with and what to.

 

[Andy Allen]

Is the adhesive and grout flexible...?

 

[Weebisto]

Almost certainly movement due to poor substrate prep. Or it could be wrong grout. Or it could be a few other things. We need the details of what tiles, how they were fixed, what with and what to.

Ok so tiles were ceramic from b&q 60x30, plywood subfloor installed by joiner, I sealed it using mapei latex sealer, fixed using Larsen std set adhesive 100% coverage and left for 24 hrs then grouted using mapei grout from Tile Giant....twice

 

[Tile seller]

Evening,
Mapei ultra colour? If so, then that's a good grout.
I wonder how thick the ply was and how well it got screwed down.
Sometimes by treading/pushing u can see the movement close/open the cracks.

 

[Weebisto]

Evening,
Mapei ultra colour? If so, then that's a good grout.
I wonder how thick the ply was and how well it got screwed down.
Sometimes by treading/pushing u can see the movement close/open the cracks.

That's another valuable lesson: never assume that a joiner has done a sound job on the sub floor - test it first