You could be sitting on the wedge incorrectly. You can place the steeper side of Gokhale Wedge to the front and see if this helps. You may also need to work on your stacksitting technique. Pull your buttock flesh back as needed and make sure your back is not swayed.
The wedge is designed to grip most surfaces, but highly polished or slick materials may cause some sliding. To prevent this, we suggest placing a folded yoga sticky mat or a non-slip rug pad underneath the wedge for added stability.