Can't get Mailchimp integration working

I can see that Mailchimp has a lot of topics, but none seems to help me. I cannot get this integration to work. I have double checked my API and list ID.

I have a Group in Mailchimp – with 4 sub-groups for each level of membership. I cannot get it to work.

I am putting:

listid::grouptitle::groupname

I’ve tried it with spaces and no spaces between the colons, as it’s hard to see on screen whether there’s space or not. I do not have logging on as it is on a live site. When I sign-up a test account, I get the S2 welcome email, and I get the admin alert email to say there’s a new user. I do not get a Mailchimp opt-in email, nor is the user added to my mailchimp list.

What am I doing wrong?

I have debugged this!

In Mailchimp on the API link there’s a record of the API calls. It had an error code:
(250) List_MergeFieldRequired

I took off all my mandatory fields in Mailchimp, and it worked. I assume that the request was chucked out for not fulfilling a mandatory field on my list sign-up form. I can now get S2 Members onto Mailchimp.

Hope this is helpful :)

Now to work out Groups.