Since the release of FormAnalytics version 4.2.1, Matomo by default limits the number of form start and submission requests to 500 for better DB performance. This limit can be increased by adding below setting in your config/config.ini.php file.

[FormAnalytics]
max_no_of_form_submission_requests_allowed={YOUR_DESIRED_NUMBER}

Previous FAQ: Increase the number of form tracking requests allowed in a single view