Hard to say without a workbook attached, but have you tried using left join instead of inner join? This will bring in all records from your primary data source. I'm guessing here, based on your statement that there is data available in your primary data source that isn't available in Tableau.
I have attached the tableau file as well. As you could see Volume Data Dummy is the primary data and Lifecycle Dummy is secondary data source. The primary key is "Ticket Id". Ticket time is the calculated field which finds the difference between assigned date and closed date. This helps me to find time per ticket id, however if i need to find monthly average time for ticket lifecycle or at status or say priority i need to have ticket id in the view and i am not able to find what is the average or median or max time.
How can i achieve this.