Board Thread:Dark Souls/@comment-24922390-20140515114651/@comment-3492791-20140515163125

The mask is guaranteed, but pinwheel drops the mask rather than it being placed in your inventory. Thus when you died and reloaded the world, all the drops were cleared from the zone. It's not like it really matters anyway. You can still get the masks from the Pinwheel Servants or Patches.