Mate I would also suggest checking the humidity of the drywall. I have had this issue even after applying 3 coats of mud. If gypcrete is being poured or you have excessive humidity before joint compound has time to cure properly you will have shrinkage. The best way to check is by going to your local hardware store and picking up a humidity prong gadget its very inexpensive we are talking 40 bucks or so. Just plug into your drywall and let it check the humidity make sure you are definitely no more then 15-17 percent.