Yes, this is what I do also. Only I use straight pins, pinning opposite sides (top & bottom) first. Pinning into the carpet pad at an angle. Make sure it is squared with a wall. Then pin the two sides, pulling slightly to remove all excess. I end up the backing (WSU) pinned to the carpet so that I can bounce a coin off it. Then I float the batt down. Finally, I float the top down and carefully pull it into place, then remove and repin the top and bottom together into the carpet with the batt sandwiched inside. Once all pinned, I baste the entire unit together in serpentine rows about 4 inches apart. Never had any slipping with this method.
Also sounds as if you have a thread tension problem.