floor tiles, grout cracking

i, recently laid quartz floor tiles in my kitchen with electric underfloor heating,
they have been down about 2 months now and when washing the floor i noticed cracks in the grout, having a closer look at the tiles i can see very slight movement in some of them when putting weight on, not enough to crack them but movement all the same.
could this get worse?
it seems to be only where the floor heating is that this has happened.
should i lift the offending tiles and relay them or just dig out the grout and replace that.
AND
why has this happened?
the floor heating is running at around 24% c morning,
9% midday
and 24 % night.
is this too much?
any advice would be appreciated

I would defo re-bed the loose tiles as it will not improve.
After the tiles were laid, it is advised not to turn the underfloor heating on in the first month, then slowly over a about 4-5 days bring the tempreture up from low to high.
But the guys on the tiling forum, could give you a few more pointers to what has gone wrong.

thanks
i asked a tiler when to start using the heating and they said a week or 2,
which i did

Did you use correct adhesive for underfloor heating?
Are the tiles laid to timber or concrete floor?

flexible adhesive,
concrete floor
i've redone this thread in the tiling forum
//www.diynot.com/forums/viewtopic.php?p=1541487#1541487