I do tend to agree with Shedman about applying a mist coat rather than dilute PVA (Unibond). New plaster, especially in a bathroom, shouldn't be surface-sealed.
Dilute emulsion soaks in to the pores of the plaster to give a good hold.
Another reason why this crops up, is that vinyl emulsion is used. Properly, non-vinyl emulsion for new plaster should be applied. New work, including new houses, always had a few coats of non-vinyl "contract paint" put on. This was only intended to last a year or two before final decoration by the owners.
The same applies to renovation work as well. The trouble is, most folk can't wait, so the vinyl goes on right away. If it's well diluted, the vinyl is broken down so that it sinks in well, and doesn't seal the whole thing up too much.
Dulux say thin with only 10% water, which I think is a nonsense. It'll just stay on top, then peel off. You should be able to see through a mist coat.