Return Visitors - Rest/Java API (1 reply)

I cannot get piwik to recognize return visitors. I am setting the _id parameter to the exact same 16 character hexadecimal string each time. However, each returning visit is counted separately and I don't get the return visitor icon. Also, I am navigating to the site from the same machine and on the same browser with the same plugins as the previous visit.

Is there anything else I need to do for piwik to recognize this correctly?