Movement in floor insulation boards

[Kr3929]

I have put down 100mm PIR boards ontop of a new concrete slab, some areas are fine but in others the boards have a lot of movement. Is it better to use sand underneath to level out or is it possible to fix in place with screws/washers? Over top will be a vapour control layer, underfloor heating and screed.

Thanks in advance

 

[Mr Chibs]

When the screed goes down, it won’t move.

what’s the movement though?
Are the gaps not filled or is it a curve in the boards??

Fill the gaps and tape up before VCL and screed.