When the form is submitted, sometimes it successfully submits and other times it just redirects back to itself with a now-blank form (the data is not submitted). So far I’ve only been able to reproduce this behavior on Chrome.
Do you have a custom validation or code that modify the posted data? Could you please share the JSON (PRO version) or XML (free version) export file and the PHP file of the form and the custom code (if you use it)?
Also, could you please try to reproduce the issue on one of the WordPress’ stock themes (like Twenty Fifteen) with other plugins deactivated? If it disappears, then you can activate the theme and plugins one by one to see which one causes the issue.