I have encountered that same problem often. I take a straight pin and scratch the paper backing in the middle, ripping it a little. Grab one of the torn edges and pull off what you can. Continue until it's all off.
Also, re-ironing it can help it release. You can try to roll a corner/point between your thumb & forefinger to see if it will begin to release. This only seems to happen to me when the Wonder Under is new. With the older stuff (really old) the paper didn't want to stay stuck to the webbing long enough to get it ironed to the fabric. However, when finally ironed down, it did release nicely. Go figure.