I created a dataset that joins 12 other datasets together and for some reason it is reporting 0 rows. I have double checked the join columns and everything seems correct. I can see that each of the 12 tables have data inside of them, too.
Given that you already double checked the join columns, and confirmed that data exists in all the tables, my suspicion is that one of the keys has a mismatched TYPE. For example, maybe one of the date columns is a VARCHAR while it is a DATETIME in other tables.
This is coming from the INNER JOIN and if just 1 of them is messed up, then the whole thing will return 0 rows.