Cracking grout

[White Room]

Dig out the grout first then Silicon

 

[guppy]

everything said seems sensible advice.I like to use a grout additive in place of water if i am going over anything other than concrete

 

[Wylie]

I think you could rake the grout and then either use flexi grout or an adflex in your grout mix
It sounds like there's movement in your studs behind the boards,i have seen plent of tiling done over ply and is still being done

 

[Time's Ran Out]

If its the grout cracking in the corner/edges to shower tray and not in the joints - then rubber Silicon seal all these.
What thickness plywood was used and was it in full sheets?
As suggest when you can supply some pictures you'll get more comments.
In the worst case scenario if you replaced the walls and retiled your best solution would be onto wedi board - tanking the corners and junctions.
Hope the sealant solves the problem!

 

[Aston]

if the crack keeps happening in the same place from the base to the ceiling, then this is a movement/structural crack...somewhere along that line there will be a joist or floor issue that is causing the movement that is causing the crack...

did the ends of the ply wood fit fully to the corner of the vertical studwork or was there any overhang? what centers was the ply fixed at 300mm 400m?

theres so many factors with loft conversions that have to be considered...you have a new suspended floor, weight of the roof, lots of new stud work etc etc...as above some pics would help solve this accurately and promptly for you

ed