By default Matomo (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 Matomo 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

Important notes:

More information about unique visitors

The number of unique visitors across the month is processed by calculating the number of distinct visitors looking at the logs for the whole month.

When you select a date range, by default there is no unique visitors metric processed by default. And if you sum up the daily unique visitors metric you will not get the total unique visitors across the date range: this is because one visitor may visit the website on several days, and when you sum a number of uniques per day you will end up counting the same visitor multiple times. Therefore to get the number of unique visitors accurately across a date range period, you cannot sum the number of uniques for the sub-periods within and must enable this feature.