I sandwich my quilts on my bed. Okay, before you groan.... the quilt on the bed gives enough texture to hold the backing very smooth and then the batting lays nice and flat, etc. Sometimes I have to pin half and then move everything, smooth it out and pin the second half. It works for me.
But, I used to get tucks all the time no matter what method I used to sandwich things together. I discovered that it was the machine I was using. I have a $$$$ Viking embroidery machine that never gets used for quilting now. I bought a vintage Singer 15-91 at an auction for $10, cleaned it up, had to buy a new bobbin case, but I use that machine for all my FMQ now.... never a tuck. I love the machine so much that I bought a second one for my summer cabin and then I bought one for my SIL who would come to my house and quilt because she had the same problem. So, maybe it's your machine. I tried everything with that Viking but it always made tucks on the back.