Duplicate Contacts not found by drop down menu on Mac

There are many duplicate contacts on my Mac, in iCloud and Gmail. From the drop down menu under Cards, clicking on Duplicates, a text box comes up saying there are 9 duplicate cards. It does not list the duplicate cards so I can compare the data for old information. There are 100's of duplicates it does not find. Why is that happening and what can I do to eliminate this problem in the future?