Nulls Show When Data Blending

Ensure that the fields are equivalent in value, i.e. verify that the field length, field case, and field values are the same. This can be done in the database or in Tableau Desktop.
 

Values in the linking fields must match exactly, including case, spelling, and white space. For example, if a field contains the state name "Washington " in database 1, and "washington" in database 2, the fields will need be converted into the exact same values.
 

Option 1

Edit the values in the data sources to be exactly the same

Option 2

Link on calculated fields that correct the values
For the above example, the following directions could be used:

1. Create a calculated field with a name like "New State DB 1" with a calculation similar to the following:

   TRIM( LOWER( [State DB 1] ))

2. Navigate to Data > Edit Relationships...
3. In the Relationships dialog create a relationship on [New State DB 1] = [State DB 2]