The only way I can avoid those pleats is to pin backing literally to my carpeting. Then put pins no farther apart than a handspan. Must have used 100.s in my King. My first ones had pleats until another quilter told me about pinning to the carpet.No more pleats. But you have to constantly look at the back. Thought I'd lose my mind ripping out one of the 1st quilts. DH hears me hollering to myself, he heads for the back door!