Jazzy Forms: some people can't send form

I have a placed a Jazzy Form at this page where people can sign up for a benefit at school.

They can choose what to eat and immediatly see what they have to pay at the bottem of the page, very convenient.

When the button is clicked, the form is sent to the user and to me. I tested it and it works. Now I get the remark from some people that the form will not send, but I recieve dozens of sent forms from other people.

So my question is, are there any known issues between Jazzy Forms and other plugins or browsers? Thant would really help me to isloate the problem.

Thanks in advance!

https://www.ads-software.com/plugins/jazzy-forms/