i wear either SPANX or the Target version "Assets" almost every workday (which are both made by the same company). when was the last time you bought them? the power panties they make now are supposed to have "no flat butt" construction. and if they are rolling at the bottom, have you tried the ones that go down past you knees but are not full-to-toes hose?
Have your Assets held up? I was very disappointed in mine. They seemed to "fall apart" pretty quick compared to the Spanx I had purchased at Nordstroms.
However, when I moved to the "hide & seek" line it's not at all like the ones at Target so I haven't even bothered with the Assets again. I like the "underware" feel vs the "pantyhose" feel, but it's really a matter of personal comfort.