When running the full duplicate search or incremental search process some duplicate records aren't pulling into either of the processes.
When running the Fast or Detailed process, anything less than a 70% match will not show in the results.
You can create a custom process that looks at match settings lower than 70% based on criteria you select. However, please note that the custom process with a lower percentage can generate a lot of records that may not be matches. Alternatively, if you know the specific records that are matches you can run the Merge two constituents process.
Steps to Duplicate
Click Constituents > add individual
Fill out the information on the add an individual screen (last name, first name, email, address, etc..)
Repeat steps 1 - 3 but don't give the new constituent the exact name of the first constituent you added
Make sure they have the same address, email address, and phone number
Click on the Constituents tab > duplicates > Full duplicate search
Click edit process and select either the fast process or the detailed process