I've had this randomly happen with 4 or 5 people now:
I see that they have a yellow tag on one or more of their co-op quests but when I go to that quest it is all grayed out so I can't send anything.
One person this happened with added me as their neighbor, still grayed out.
But when I added them as my neighbor I was finally able to send their items.
Not sure if I'm doing it right but will try to include a photo.
