Unable to use minecart with hoppers to gather items from Nether Portals

Preamble
Create a portal in the overworld with a dropper on a clock to throw items through the portal into the Nether. At the corresponding loc in the Nether, create a portal and seal off both faces with glass or whatever. Directly underneath the portal's floor, place two minecarts with hoppers to retreive the items, on tracks.

What I expected: Carts retreive items as per items on all other block types.
What I got: Items bob on portal floor.