Workflow not working when trying to update values

I am trying to update 3 fields in contact form through my Last completed Phone call activity. I have a custom button on my main CRM Ribbon "Add Activity". When a contact is selected and this button is clicked a phone call activity form opens up with due date, and few other fields auto-populated  When "Mark Complete" is clicked the workflow should run and update the due date field in the Contact page from the completed phone call activity. My workflow is not working. It does not proceed further after the first if condition "If Phone Call-> Activity Status = Complete". Although the status is completed. And the workflow is in waiting condition. Any help would be greatly appreciated