Dab Not Sticking To Thermal Plasterboard

[Renovation]

My plasterer has dabbed some Gtec XPS boards and whilst the dab has stuck to the wall the boards can be pulled off cleanly.

My supplier said the dab ought to be spread over the whole board as well as the wall being dabbed - plasterer says he's never had a problem.

The celotex boards come with a paper backing - which have stuck but these Gtec boards have a polystyrene back - the only reason I bought them is they are available locally whilst the celotex ones had to be ordered in and I didn't have time to wait.

[Renovation]

i've just called Siniat Tech Line - they say thermal boards only need more dab than std boards.

But as my reveals were around 50% covered and they still haven't stuck that's no help.