Why have lines appeared in my plasterboard ceiling?
When you say that the plaster board has sunk, could it also be that the compound between the boards has shrunk, and created the illusion that the board had dropped? This is an important distinction. If the plaster board is in fact dropping, you have a fastener problem, and this needs to be addressed immediately by screwing the board tightly to the lathe or joists that it is attached to. If, however, the compound between the joints has lost some mass due to dehydration, this is cosmetic; but reveals a potential costly problem. Some of your other answers refer to taping of the joints, and it is possible the contractor simply applied compound between the joints without tape. This is a big no-no and a legitimate cause of action to have the ceiling refinished properly by the original contractor.
