Can you share your forum URL or try checking the browser development tools to see whether any request is made to your Matomo server under the Network tab?
Also take into account that you might have opted-out of tracking on your main browser via cookie, you might be serving Do Not Track header or might be using an ad blocker extension that prevents Flarum from making the request to your Matomo.
If you have my extension enabled and have filled all settings, then the javascript tracker should be visible at the bottom of the HTML generated for every Flarum page and you should see attempts to make requests to the Matomo server in the network tab of the development tools.
If the URL is malformed or some other setting is invalid, this should also become obvious in the development tools where requests will show as failed with the details.
