Why is the Last_Activity_Date_vod__c field null on the TSF_vod__c record?
The Last_Activity_Date_vod on the TSF_vod record is set to the value of the Call_Date_vod__c field on the Call2_vod__c object. The Last_Activity_Date_vod field on the TSF_vod record will be null if there is a change made in the current year to a call that was recorded in a previous year. If there are no changes, the Last_Activity_Date_vod will remain populated with its current value.
CRM Help Documentation: Working with Territory Fields