A hopper minecart doesn't pass items to a shulker box. I don't know if this is an intended game mechanic or a bug and since I didn't find any info on it, I posted it here.
I tried it with the minecart on a rail and without the rail, with the shulker box facing different directions, but no results...
Hopper minecarts only take items in, they can't push items out. Use a regular hopper to pull items from the minecart.