Form doesn't work on diff sites with same theme & versions of wordpress/plugin

Hello,

I love Easy Mailchimp Form, which works like a dream on one of my sites, but cannot get it to work on another. Both sites use the same theme, similar plugins (I tried deactivating them all to see if that was the problem, to no avail), and same version of WordPress. When I enter in the required fields into the form and hit the submit button, the page refreshes but nothing happens. No success or error message, just nothing. Any idea what the hangup could be?

Thank you!

https://www.ads-software.com/plugins/yikes-inc-easy-mailchimp-extender/