If you're talking about the stretch shadows, that comes in part from not stretching the cloth enough end to end when setting the feather strip, and from placing the staples right on the stretched portion of the cloth when stretching it to staple it.
Take a wet wash rag, single layer it on the rail cloth, and rub the rail cloth, top and bottom back and forth to loosen up the fibers of the cloth, then it'll loosen up a bit and a lot of the shadows will go away. I'm not talking about a "wet" wash rag, wring it out tightly first, then use it. Don't be afraid to apply some pressure on the underneath side of the cushions against the cloth, as that's where you need to apply most of the pressure as you rub the cloth back and forth anyway. Trust me, with a damp rag, the cloth will loosen up a bit.
Glen