if it is (pulling up in the center) while quilting --to an extent that is normal- just like doing embroidery & applique---as you quilt the fabric (draws up) some---which is why the backing/batting needs to be larger than the top-
if it is a big problem---a couple things === is your backing pieced? do you have one seam running vertically down the center? if that is the way it is loaded with each turn of the bar the center is building 4 layers of (bulk) compared to the one layer on each side---when possible seams should be horizontal so they lay flat & even along the bar.
also-about the direction of the fabric-
the lengthwise grain has less stretch than the cross width---so i can make a difference if the salvages are top & bottom (crosswise grain) or side to side (lengthwise grain) going up & down do a (pull test) & see which direction your fabric stretches most in---salvages should be removed though---they are a tighter weave and effect the (lay of the fabric)