Painters advice, bubbles on wall

[AidanGee93]

Painters advice:

Using a water based paint for my walls, first coat went on lovely but when dry I noticed on most of the walls little air bubbles (some as big as a fifty pence piece). Let it dry and I sanded them down which worked. Upon painting second coat most of them bubbles have reappeared?

Id love to know you're thoughts/advice for treating as I'm thinking the walls where never plastered...just plasterboarded and joint taped and Mabey the paints seeping behind the paper on the plasterboard causing bubbles?