When you create a new custom report, the data for past dates won’t be available by default and you will only see reports starting from that day you create the report. However, if you want to still get the data for reports in the past, you can

  1. Invalidate the historical data, and
  2. then re-process the reports for all your websites. If you have setup auto-archiving then the command to re-process the reports will automatically run within the next hour or so. Alternatively you can manually re-process the reports. Depending on the amount of data in your Matomo (Piwik), this command may take a while to run and you may want to adjust the amount of days (and seconds) you want to reprocess.
Previous FAQ: Can I pivot Custom Reports?