I hammock camp all the time. I use an underquilt since I thrash and toss and turn when I sleep and the underquilt generally stays in place. Pads do not.
Did you ever try a double layered bottom hammock like a Blackbird where you can put the foam pad in between the sleeves? My pad stayed in place much better. Some will even use velcro to keep in place or apply something that when dry will give more friction.