First, verify that the accounts have not been marked as exclusions.
  1. Navigate to one the duplicate accounts
  2. Click on to the Other 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

If the issues is still unresolved, Contact Support and reference this article. 


Workaround:
When you know duplicates exist, the two accounts can still be merged by clicking Merge Role and searching for the matching account.