Been happing probably over the last few months when trowling up with a bit of water little patches of skim just drag away like the 2nd coat is coming away from the 1st coat if u try filling it it keeps dragging away only way it seems to get rid of it is to take of the plaster back thinner to the board in the little areas happens on plasterboard and not every time and not every wall so I'm thinking is it the plasterboard