Is there any way you can clamp your backing to something while you put the batting and top on? I had the very same problem until I started clamping everything as I made the sandwich. I have two cutting tables that I put together to center and clamp each piece of fabric, batting and quilt top. I use small carpenters clamps in the corners, centers and small clps all around the edges. This takes out all the wrinkles and makes things pretty flat. There is a YouTube video about this where the instructor uses a thin sheet of plywood over a regular table to do the same thing. She uses those binder clips from the office supply store to secure hers. You can have part of the sandwich hanging over the edge, just pin (or spray, although I've never used spray) the whole middle, then unclamp it and pin the edges. I mostly make queen size quilts and it has worked well for me. Good luck!