Tree showing without tree behavior / as flat list

Using 7.6.1 and JDK8

I have two Vaadin applications.
One is simple and presents a Tree (like
) as expected.
Another Vaadin application is more complex and the presentation of the Tree is a flat list. No decorations. No expand/contract icons.

Any ideas why this may be the case?

With Decorations (simple application):

Without decorations: