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I have found some cracks in the grouting of my floor tiles, only been grouted a few days.
To the best of my knowledge I have followed all the advice given on this site and the instructions on all the gear used.
Everything looked great until this morning when the Boss pointed out there was a crack in the grout, along the edge of three tiles, it looks as if the grout has shrunk and pulled away from the edge of the tiles, can the heat cause the grout to dry too quickly and shrink, or can it be down to a bad mix (too wet or too dry) or is there something else I could be doing wrong, I am using Bal micromax 2 flexible grout.

Could be a number of reasons either on there own or factored together.

Sub floor not sufficiently strengthened before laying the ply. Flexible grout only has so much flex in it.

Grout could have shrunk back due to weather, although not come across this myself. If that were the case then one would expect more cracking.

Larger format tiles which were not back buttered or checked for adhesive spread when layed.

If using rapid set adhesive, it has become too dry for use but still been worked with.

Thanks for reply,
I screwed down all floor boards and did the pint of water test before screwing the ply to the floor, the ply was screwed every six inches, and longer screws were used when the boards lined up with a joist.

All the tiles are 300 x 300, just googled back buttering , I spread the addy on the floor with the notched trowel and laid the tiles.

When laying the tiles that have cracked I was using rapid set adhesive, but as said in an earlier post laid ten tiles and addy went off, so had no choice but to throw it away.
still got 100 more to lay so if i am doing something wrong would like to put it right

Where the grout has cracked, stand on the tile and just jiggle on it to see if you can spot movement. If there is too much movement or flex in the floor you will see the tile move slightly as you move weight from one leg to the other.

The grout may have gone off too much to be used. I am sure that the grout you used is the quick drying one.

You do have the option of taking the tiles up by the cracked grout and laying again.

As a rule with quickset, if you knock up one third of a bag or a quarter and mix by hand, this should be a good amount to cope with using within its workable time.

Half and full bag mixes are best left to us pros in colder weather and when a good area can be laid quickly.