My client based in Chicago has been having issues with some of their users from other countries not being able to submit a form, mostly the UK, Scotland, and Germany. They have had successful submissions from the UK, South Africa, Romania, Canada, Egypt, Australia and others.
This form is their most popular form on their website with 1362 entries. This form is their most complex as well with 3 file upload options, Captcha, multiple choice options, and address fields and so on.
There have been around 5 users unable to submit the form. One of the users said "your web form failed (it doesn't seem to be working)".
Other than that I don't have much more detail on this issue, but any issue with this specific form is a BIG deal for my client and they are questioning the integrity of the form and the Gravity Forms plugin (which I know is the best form plugin out there for WordPress).
So if there is anything that you know of that might help, please let me know.
The form in question is on this page:
http://medallionmediagroup.com/submissions/
Thank you!