Sometimes if an item is dropped by a merchant the item just sits there and cannot be picked up. Typically happens when the game puts the item on top of an already occupied tile.
If you move the thing that’s underneath it, the item should fall to the ground and you should be able to pick it up.
Try moving the town flag, stuff has got stuck up on top of it for me…
I had a trader drop 10 cloth that wouldn’t get picked up, and usually loot from goblins won’t get picked up. I found if I made a stockpile under said items and then removed it the items would be moved to the correct stockpiles.
(that was alpha 10.0, goblin drops seem to get picked up now)
You can also build a ladder to items which are “floating” in the air. That way workers are able to climb up and grab them. Works also with workers who are stuck by themselves.
Don’t make the ladders too long because if the items are on the ladder you can’t pick them up.
But you can delete the ladder to make the item fall, then you can pick it up.