The duplicate check process is based off of a scoring system. If the accounts don't score high enough, they will not be found as duplicates. For more information on how this works, please read the article The Duplicate Report Vs. One Off Duplicate Check

First, verify that the accounts have not been marked as exclusions.
  1. Navigate to one of the duplicate accounts
  2. Click on to the Account Settings page
  3. Click View Duplicate Exclusions
  4. If the other duplicate account is listed, remove them from the list
If neither account has the other one listed as an exclusion then you will need to verify that the accounts are not linked in a relationship.
  1. Navigate to one the duplicate accounts
  2. Click on to the Relationship page
  3. If the accounts are linked in a relationship, delete it and then they should be found as duplicates
When you know duplicates exist but the two solutions above do not work, the two accounts can still be merged. How to manually merge accounts