This is an uncommon issue and one that is easy to overlook if you’ve recently deployed Sitecore into production.

But you find that the persistant cookie “SC_ANALYTICS_GLOBAL_COOKIE” which Sitecore uses to track contacts across sessions in the xDB is missing. This causes a number of issues, but the main one being is that if a visitor returns to the website, a new contact is created as Sitecore cannot relate the two sessions using this unique visitor id.