Running workflow on Assign across Business Units

Hi All,

I'm in need of some assistance and am hoping that you will be able to help me out.  

I've created a workflow that I'd like to use to email a user when a Case has been assigned to them.  The email is sent from the user that has modified the Case to the user that the case is assigned to.

This works perfectly well when the assignment is going from one user to another if both are in the same business unit.  It also works when going from a child Business Unit to the parent business unit.  It doesn't work when assigning the Case from a parent business unit to a child. 

Just to make it a little easier:

Parent -> Parent  :  Works

Child -> Child : Works

Child -> Parent: Works

Parent -> Child : X 

There are no errors, the process just doesn't start,  it doesn't get added into the Background Processes.  This also doesn't seem to be a permissions issue as I have been testing using two Admin / System Admin privilege accounts.