Hey again Beatriz,
A quick follow up on this one. The issue is most likely related to the plugin that you're using uses validation via ajax to see if the required fields are filled out or not.
In this case in order for your form to work properly you'd have to use "Refresh PopUp or do nothing (use for Ajax Forms)" in Form Submit option which is available only in premium version of the plugin unless the plugin that you're using has some option to change to non-ajax validation.
Hope this helps :)
Best regards,
Bojan