When the upholstery is tight, as you describe I, as said above too, used pure silicone spray on the foam. It will allow the material to slide. Does the Upholstery have strings? Are you using hog rings? The hog rings will pull on the strings or rope to attach it to the springs, which will give you the shape that you are looking for. I have seen guys who don't have hog rings or hog ring pliers use nylon zip ties to pull the material and hold it into place. Sometimes there are metal rods that slide into pockets of the upholstery and where the cut outs are is where the hog rings pull from.