A Campaign Goal is the desired outcome for the given initiative within a Campaign. Every ActiveDEMAND Campaign has a goal-setting section that allows you to track one or more desired outcomes for the given campaign. Common goals are 'filled out this form', or 'purchased', etc.
A contact history item created on a contact triggers the check to see if that contact 'signal' pushed the prospect across the goal line (and completes a goal). The only way a field change will complete a goal is if the field change is done manually (as that creates a 'user changed' history item on the contact. Thus if a field is changed via any other means, the only way this contact will be checked to see if it completed a goal is when a history item is created for that contact.
No contact will complete a goal until the contact does something that causes the creation of contact history item on the timeline
An easy way to diagnose this, open the contact that you believe should have completed a goal. Check to see if there is anything on the timeline that happened while the campaign was active that should match the goal query.