Just after xmas I had all the walls plastered and although it dried out everywhere for some reason a patch took ages to dry out and was still a bit damp but being under pressure from the other half I painted over it all and after a couple of weeks the damp part started to flake and crystalise ..now last week I sanded it all down and repainted but its started to flake/crystalise again ...its not down to any leaks /damp position as its in the centre of the wall ( not external or any pipes behind) is this normal to still be drying out after six months and how do I cure it