Hey Michael,
I took a look at your site at the index.php address and I was similarly seeing that the subscribe button was not working.
Generally issues like this are related to a Javascript conflict on the page, so you have a few options in moving forward.
One option would be to use our embed code sign up form, which MailChimp provides to you for each list in your account. The embed code allows you to generate a version of your sign up form with no Javascript, which would allow you to avoid any potential Javascript conflicts altogether. However, the interaction would be slightly different in that subscribers are directed to another page, rather than the on-page validation messages, which Javascript provides. I’d definitely recommend this first option as a starting point, as it allows you to more definitively say whether this issue is in fact a JS-related issue.
Here’s a link to find that embed code:
How can I add my signup form to my website?: https://eepurl.com/gOGN
And to generate a no-JS version, you can use the “Classic form” and disable javascript in its options, or use the Naked Form.
The other option would be to troubleshoot what specifically is causing the conflict. To do that, your best bet is to enable the plugin’s sign up form, then disable all other JS on your site. Slowly re-enable each bit of Javascript code and continue to test the sign up form after each re-enabling. When the sign up form stops working, you’ll then get a better idea of what other plugin or piece of Javascript code is causing the sign up form to no longer work.
If you have anymore questions, just let us know!