I have played through the game 3 timees now, and each and everytime it never registers when I pick up the Witch's Hat.  I was wondering if there was a fix for this as I have tried selling and buying back the hat, equiping it, and putting it in and removing it from storage and it is the only item I need for the achievement.

 

Have you looked under your collectibles tab to see if the witch outfit says 3/3? Have you checked every other item under the collectible to ensure nothing is blacked out with "???" for an name and that every outfit says 3/3? There is a bug to where even after you pick up everything in the game the map still thinks there are 3 pieces of equipment in the school and six pieces of equipment in Kyle's yard but this won't keep the trophies from popping.

 

There are quite a few hidden treasure chests in Kyle's yard too, you have to be gnome size to reach two and two are hidden in the tents there. Have you got all those? On your map, select to display treasure and all it should show are 3 pieces at the school and 6 at Kyle's house(these are bugs) and nothing else. If any other place says there's treasure then you're missing something and some items can only be obtained during a certain mission, for example when you have to choose to fight Kyle or Cartman there's treasure in that room, including a sword that you must pick up before you start the fight or you'll never have another chance to get it.