Kyia, it sounds like you may not have it pin basted enough. You should have a pin at least every 4" or about every "hand" width as I measure it. If you firmly smooth your backing, then either clamp it to the table top or tape it to the floor to hold it securely, then lay your batting on it, then the top and pin baste it all over.
Then, I lay a section of the quilt under the foot on the machine and smooth the back as best I can in the little section I am working on. I don't use clamps..I fluff & stuff the quilt into the machine. I first stitch in the ditch to secure sections of the quilt at a time before I start doing more detailed quilting.
Good luck!