What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Basically it does not really matter if you use HTTP or HTTPS the form should work the same for both. The difference is that data served over HTTPS protocol are encrypted while for HTTP it is not.
I also tested your form with both HTTP AND HTTPS and submit every time (test make in Chrome on windows). It take a few second before it redirect though. This something that happens sometimes for long form.
If you notice the problem still persist on your hand trying clearing browser cache and also make some test on other browsers.
Do not hesitate to contact us if more help is needed.