Mailchimp extension not pushing new Civi group members to the Mailchimp list
The instructions page you linked to is for an old version, which you must be on if you have 1.8.7. 

I wrote version 2 of this extension and it's fairly a complete rewrite, so while I can't promise it solves your problem, I can say that no further development is going to happen on 1.8.x.

You need to update to v.2 before the end of 2016 as Mailchimp are apparently pulling their old API, so your integration will probably stop working. Veda have most of my code "merged in master" (means they've accepted it) but have not published a release.

I have published a release, though, at https://github.com/artfulrobot/uk.co.vedaconsulting.mailchimp/releases 

It's flagged with a red beta tag at the mo because it brings in an experimental webhook thing for WordPress users, but I'm guessing you're on Drupal, so you can ignore the beta bit - I have been running this version in production for a few months now. But you should see the README as the new Mailchimp API changes a lot. I'd definitely recommend a backup of your Mailchimp lists ("exports" from Mailchimp) and your CiviCRM set up before you go upgrading. Of course you would anyway :-)