Can't get MailChimp plugin to work

I have set up a new MailChimp account and installed MailChimp Integration and PopUp Pro. In MailChimp settings everything looks OK, I have added the API Key and it says Passed. In PopUp Pro I have created a popup and embedded a MailChimp Form asking for First Name, Last Name and Email, the last of which is required.

In MailChimp itself I have created a list (which is identified and chosen in the plugin) and the fields in the list are FNAME, LNAME and EMAIL.

When I fill in the PopUp form on the site, the details never appear in the MailChimp list. Nothing I can think of makes any difference - I have deleted and recreated the list, changed API keys, reinstalled the plugins and deactivated all other plugins, but still I can't get the plugin to work.

I presume it's a config error on my part, but I cannot work out what.

Please help! I have enabled access to the site if you need it.