Cannot use the field value of an in-scope object to update the corresponding field value of another in-scope object

I have been carefully working through the seventeen Rules for D6.x video tutorials provided by Johan Falk at Node One (http://dev.nodeone.se/en/learn-rules-d6-with-nodeone). Everything was working perfectly until video #5 - "Object Loading and simple CCK field Population". No matter how I tried, I could not get the rule created in this exercise to work. The rule is supposed to copy the signature field contents of the user's CCK Profile content type into the signature field of the Story content type, conditionally on the Story signature field being empty. The signature fields were created with machine names "field_story_signature" and "field_profile_signature" (all exactly as per the video). At every stage of building the rule, I carefully compared what I was seeing on my monitor with the video version. There was never any deviation between my version and Johan's original. Only mine has never worked. I even re-created my localhost sandbox site online at Hostmonster, just in case there was some kind of configuration issue involved (my development system runs WAMP on Windows XP and Hostmonster runs Linux on x86-64 hardware).
The problem was identical on both platforms, which I think rules out any configuration issues