If possible, can you share a packaged workbook (.twbx) which is displaying this issue? This will make it easier for us to determine what is happening.
In the meantime, I would check the information here: Blending Data on Non-Matching Date Fields | Tableau Software
When you use two data sources in one view (data blending) you're basically creating an ad-hoc left join on whatever field you use as the linking field. My guess is that your two date fields don't match exactly... there's not a corresponding entry in the 2ndary data source (orange check mark) for every value in the primary data source (blue check mark), and so Tableau isn't able to bring anything back because it's only looking for that match.
Honestly this is notoriously difficult when blending on date fields and so if you have any other field that matches between the data sources like a product or order ID, you will probably be happier trying that one. Or, if you can join rather than blend the data sources, you may have more flexibility. But if you must blend on a date, hopefully the above article will help.
Also, you can check out the below for more info on how data blending works and common pitfalls: