wpf TreeView Expand all children by using ViewModel

wpf TreeView Expand all children by using ViewModel

I see there are some questions related expanding all children using
ViewModel-s I've tried to follow some of them to solve my problem, but
without results.
The problem is when I'm setting IsExpanded(in viewmodel) to true, the only
top parent is being expanded, I've added TreeViewItem.Expand event handler
to my TreeView and set a breakpoint in the handler.
I'm waiting to be paused on breakpoint in the TreeViewItem.Expand event
handler, but it is happening only once: after top parent IsExpanded
changing.
What is wrong, what I missed?