A quilt cover that will not sit square is rarely the quilt’s fault. It is one of three measurements the packaging leaves out.
Drop is how far the cover hangs below the mattress edge. Under twelve inches on a double bed and it rides up every time someone turns over. Ours are cut at fourteen.
Inner ties hold the quilt in the corners. Four is the industry minimum and not enough; eight, at corners and mid-edges, keep it flat.
Placket depth is the overlap behind the buttons. A concealed placket of at least three inches stops it gaping. Ask any maker for those three numbers — if they know their product, they will have them.