Seeing the data here would help a lot and/or letting us know what you have tried. I suspect the view desired could be achieved with a join between your orders received and fulfillment tables on order id.
Is every order in the view fulfilled in the same month? I suspect not so how/what should Tableau do with the orders fulfilled in a following month (or not yet)? They are two different views that I imagine - quantity of orders in a month and quantity of fulffiled/shipped in a month and these are two separate views in a dashboard.
Do you have a workbook that contains data that can be shared? Also - what version of Tableau are you working with?
Check out our Workbook & Calculation Library as well to see if there is any example in there that may help.