How big are the remaining pieces? Here are some ideas for trying a few more things before adding new drywall "mud," which I would avoid:
Here is an excellent post by Peace Painting that will help: http://www.hometalk.com/133617/as-far-as-removing-the-wallpaper-this-was-an-easier-one-best-charles
Post a pic...was the wall primed first? You can try windex...spray it one, let it sit for a couple of minutes then light scraping. You could skim over it but it's usually a bad idea....especially if there is any mold on the backside of the paper.
Cap full (full laundry load size) of liquid fabric softener to a gallon of water, spray/spong on to soak, should peel off. Hot tap water works better than cold with this. Just rinse walls with sponge and plain water after to remove fabric softener residue.