If this is a customers house express to them first that it is not caused by faulty drywall work (I hate taking the blame). Like the prior post said....stop the movement. Perhaps the frmaers left something off accidentally. Sometimes when ceiling joists are too long they need a little extra. NO drywall trick is gonna work here. Mayber temporarily. check to make sure the tape is even mudded. is the crack down the center or on both sides of the seam? In either case tear off the tape.Durabond the crack. Retape and finish. If that dont work......FRAMING