New Plastered ceiling on new house

Make sure all paper is removed.coat with blue grit leave for 24hrs replaster

Will have to chip off and do again. Try using an adhesive glue (PVA) before plastering.

Could be lots of reasons Wallpaper left on the ceiling or No treated with PVA Ceiling was wet after palstering

It's because the plasters aren't doing it well... They are not experienced and are mixing plaster in the wrong quantity or the plaster is expired. I have 2 plasters with many years experience. Ask the money back from and ask me for a quote, I can remove and refix with my team

I agree with the above comments and also the possibility the ceiling was painted with a silky/oil based paint which in my own experience needs steamed off before work starts as the plaster doesn't take to it well at all.

There is only 2 reasons why plaster will fall of a ceiling, 1, they didn’t apply unibond know as pva all over the ceiling or lack of it , and 2 , the plaster must of been well out of date

In your title you say new ceiling on new house? Can I assume that they have not tried skimming over Artex?

The only way plaster would fall off like that it that there was still paper on the ceiling and or it was prepared properly. The ceiling should be free from paper and then Painted with knauf betokontakt which is a think gritty pink paint and then left for 18-24 hours to dry before plastering. Even if the bags of plaster are out of date it doesn't mean the automatically fall off, It will certainly set faster and have crappy hard bits in the bags making it a nightmare to use. What you want to hope for is that some paper has been missed and then its just a case of scrapping of the bad parts painting you ceiling with knauf betokontakt and then re-plaster it. But if the haven't PVA/Painted the ceiling honestly you would be better re plaster boarding. Don't worry to much about that though it doesn't take for ever it can usually be done in 4 hours