Where can I find the Wall of Flesh loot?
Solution 1:
The Wall of Flesh doesn't drop its drops in the standard way. Instead, when it dies, a box of demonite/crimsonite bricks forms in the air with the drops inside it. Make sure you're checking in the air in the underworld, and not just along the bottom.
Solution 2:
The items are placed inside a square container made of Demonite (or Crimtane) blocks to make sure nothing falls into the lava. It will be located in the exact same spot the Wall of Flesh died.
Solution 3:
It should be very obvious where the loot box appears, so if it didnt pop up, it may have been a glitch. Typically, it will spawn a square of blocks as stated in the other answer very close to the wall's horizontal axis in hell upon dieing
Did you perhaps kill it offscreen? If you can recall, try to find the exact location it died.
Either way, no loot = you will want to kill it a second time...some of the items you need for your now hardmode world are contained in that box.
Solution 4:
If you died under the box, just before you died it might have dragged the loot through the box. Because you died, it would have fallen into the lava instead of your inventory. If this is the case, you will have to fight it again. Of course, if you have a Hell Bridge under the Demonite/Crimtane box, then this couldn't have happened. In this case, it was probably just a glitch and you'll have to fight it again. Either, you'll have to fight it again.