gtk.TreeView.set_model(TreeStore) expanders do not seem to work?

["Daniel B. Thurman" <dant cdkkt com>]

In my application, I am populating a treestore with
new children, I then use the following method:

self.treeview.set_mode(treestore)

Apparently the treeview does not show expanders
where there are new children.

Is there something I need to do to show the expanders?

I have tried, with no effect:

self.treeview.expand_row(path, True)

Any suggestions I can try?