After fusing, let it sit until the glue has rehardened...the longer the better and at least a half hour (humidity will affect the time it takes to freely release). I leave mine alone for a couple days.
Since the web is not sticking to the fabric (and you wouldn't be shredding it otherwise), your iron is not hot enough. You want a cotton setting, dry iron for 7-10 seconds. Overfusing will melt the web into the fibers and prevent a good join to the second fabric, so more is not necessarily better.
I use Wonder Under exclusively, have never had any trouble with it, and use it for almost everything I do in the art quilt world. For the best tips you can find, see Laura Wasilowski's suggestions here:
http://artfabrik.blogspot.com/p/fusing-tips.html