When you say the "mapping not happening", can you be more specific about your setup and symptom?
As Otto suggested earlier (in this post) , the Strategy automatically takes the class key of the apply-to class as the value of SubjectID, so the following information would be very helpful:
Do you have keys defined for the strategy apply-to class? If so, what are the keys?
Have you tested your strategy using the Test Run panel (available as a side panel on strategy rule)? If so, what's your setup and result? Are you able to see the mapping from CustomerID to pySubjectID?