How to add listviews to a horizontal box at run time and display correctly?

I want to add multiple listviews as childs to a horizontal box at runtime. However; when I do they stack on top of each other instead of next to each other in a row.

I have created a widget blueprint that contains a listview inside of a vertical box. I then construct a number of these with a for loop inside of a horizontal box within the main ui widget blueprint using the AddChildToHorizontalBox node.

Which part am I missing?