By default Piwik will not process the number of Unique Visitors for Years and Custom Date Ranges (for performance reasons). The query to process the number of unique visitors is quite costly and takes seconds or even minutes to run on a High traffic Piwik server. You can decide to enable or disable the unique visitors metric for specific periods by adding the following in your config/config.ini.php

enable_processing_unique_visitors_day = 1
enable_processing_unique_visitors_week = 1
enable_processing_unique_visitors_month = 1
enable_processing_unique_visitors_year = 0
enable_processing_unique_visitors_range = 0

By default, unique visitors are processed for days, weeks and months (they are set to 1). You can for example enable yearly unique visitors by setting enable_processing_unique_visitors_year = 1

Note: processing the Unique Visitors metrics is using the Piwik logs. This requires that the feature “Delete old logs” is not used on your Piwik server.