Migration assistant refuses to Assist my Migration !

My old Mac at work is a mirror door G4 powermac running 10.4.11. I
have just acquired a better spec G5 cheesegrater Powemac also running
10.4.11. I have the old mac connected in Firewire mode and Migration
assistant on the G5 goes through all of the menus until I hit the
final "Migrate" button, which responds - then nothing - nada, zip...
it just sits there doing nothing. I can go back through the menu but
as soon as I get back to hit the migrate button - still nothing

Anyone got any clues as to what may be causing this strange
behaviour ?

Thamnks for any help