This can be caused a few different things either in the Matomo settings or in the setup on your website or web server.

  1. If all of the different websites are set in Matomo as the alias URLs then Matomo won’t track any of these as either outlinks or as referrers.
    If we take for example the following example websites: example1.com and example2.com

    If example2.com is added as a website in example1.com‘s settings in Matomo (See screenshot below) then any clicks on links from example1.com to example2.com won’t be tracked as outlinks:

    Likewise, if example2.com has example1.com set as a URL for itself, then any links clicked on from example1.com to example2.com won’t be counted as a referrer. For example if your settings for example2.com were as follows:

    As we can see from the tooltip in the Matomo settings for the websites (Administration > Websites/Measurables > Manage > Edit site), it specifies that any aliases added here will not appear in the Referrers > Websites report:

  2. If your website has links set with noreferrer, for example if using rel="noreferrer" then no referrer information will be sent to the website you are linking to. If you are using some other tool such as dereferer.org to strip any sensitive information from links between your website, then this can also cause no referrer information

  3. If your website has a redirect setup incorrectly (For example if you are linking to http and your website automatically redirects to https without forwarding the referrer information correctly) then its possible that there is no referrer information present when users are clicking on links between the websites.
    If this is the case, then it will be reflected in the Referrers > Websites report, as this would likely impact all referrer information for your websites.

    There is some additional information regarding this in our FAQ: Visits from search engines or campaigns are not recorded, Matomo shows “There is no data for this report” for all Referrers reports.

If this doesn’t help solve the issue, you can find additional relevant information in this FAQ: Matomo doesn’t track Downloads and clicks on Outlinks on my website.