Users in subordinate organizational units don't see workitems

[Ullrich, Ralf]

Hello fellow workflowers,
 
I have a situation which I don't understand. Maybe someone can give me the
answer:
 
I have an org structure with one top level org unit consisting of a few
subordinate org units, each with a few users.
 
Now the top level org unit is assigned to a single step task. I somehow
expected that every member of a subordinate org unit is a possible agent of
the task as well, but this doesn't seem to be the case.
 
Any idea ?
 
regards
Ralf
 
P.S.: I'm on 4.6C.