This means that max_input_vars is not set high enough for the form/environment. If your forms include multiple pages and have a lot of fields in them you may need to increase your max_input_vars to at least 3000 in your PHP.ini and/or wp-config.php settings. We have seen some client forms that are very large that required 6000.
Gravity Forms/System Status/PHP/Maximum Input Variables (max_input_vars) will show you what you have set for your site.