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A couple fo things to double check - are the fields that look like they have the same names actually the same datatype and the same capitalisation/spelling? Sometimes an ID field might get interpreted as a string in one data set but a number in another, and Tableau won't be able to join/blend them. Similarly Id is not the same as ID.
If it isn't this simple, might be worth attaching a copy of the two files you're trying to combine.
PS - am also moving this post into the main Forums (Forums ) area of the Community, where it will attract more attention.