Problem:

Duplicate Check does not find any duplicate records, or finds some but not all. Some records that I know are duplicates do not come up as a duplicate search result.

Answer:

Duplicate Check will search all records to find duplicates, but will only display active records as result. Duplicate records that are disabled (inactive) will not be shown.

What is considered an active or disabled record can differ per organization. In the Entity Settings, at Disabled Records, you can indicate which records should be regarded as disabled or active for the duplicate search results.