Did I few floors about 6 years ago in a house and had problems with the dining room. Carpet had been down for a couple of years. It was already screeded by the plasterer and was good. I rubber down and feathered it. No damp issues either. Laid the floor and a few others. This year a bubble appeared in middle of the room. Damp reading of 1000 on mms. Turns out there was a wall in middle of the room and it wasn't filled in with a dpm. So they dug it out and refilled it the right way. I've had to screed it and then retile the gap. Nightmare as the new Karndean knight tile was a fraction smaller then the ones down so had to mess about to get it to fit. Run out of strips too. So morale of the story check the history of the floor if you can. In fact got a question about filling in but will put it in the right section.
