I have just had one of my walls skim plastered after I erected a stud wall.
This was last Thursday, the wall looked good and I painted an undercoat
yesterday (a cheap trade magnolia with some water added as suggested by the
plasterer).

I have just put on a second coat today (paint seemed dry) but on one small
area of the wall (about 2x5 inches) when I run the roller over it a small
amount of the paint underneath lifts off. I've just spent a good half hour
trying to get this painted but have had to give up in the end with this
patch still bare plaster.

What am I doing wrong?