Painting on primed plaster...flaking?

I have recently stripped paper from a wall in my house and having sanded it, I then applied a watered layer of paint to it.

I have found that whilst rollering the first layer of finish paint onto the wall, small flakes of the dried primer (watered down emulsion) are coming off the wall and/or causing parts of the primered areas to bubble??

What is this a sign off?

Not washing off the glue residue

Nige F said:

Not washing off the glue residue

Click to expand...

Ahhhhhhh!!! So stray bits of dried PVA from plastering the ceiling could also be a culprit?