In rare cases, Matomo (Piwik) will show “no data” for a subset of data, for example for one day, one week or one month, while other periods will work fine and report all data as expected.

How to solve this problem?

Congratulations! you should have now restored the missing data and likely prevented this issue from occurring in the future.

What causes this issue in the first place?

In short, there could be multiple reasons and it’s difficult to know.
Unless this was an ongoing consistent issue it wouldn’t be worth investigating as this may be the only time it happens.
But if you continue to have this issue with invalid reports or incomplete reports, we recommend that you confirm with your team that you have setup the crontab with the MAILTO line, so that you get an email whenever the crontab fails.

MAILTO=your-email@company.com
1 * * * * php /path/to/matomo/console core:archive --no-ansi >> /path/to/private-logs-folder/cron-matomo-archive.log

In this crontab entry replace your-email@company.com by your Matomo system administrator team email. If there is an error during the script execution, the script output and error messages will be sent to this address.

It is helpful to review and monitor these errors and take actions to prevent these missing data reports issues.