Mailchimp Integration not working

I have total 26,500 registered users mostly on above blog but also and I want to send emails to these people preferably with Mailchimp rather than using eNewsletter. Have tried to sync these users into my mailchimp list but mostly unsuccessful. Only 646 of these users are on this list. My Mailchimp API has been verified successful. What is also confusing is approx 50-75 new users total arrive every day but are not joined. is using only your integration plugin plus mailchimp optin widget in sidebar. I tested a random user with this widget. Successful optin and this user appeared in my mailchimp list. Checked error logs. No errors. This problem is ongoing so please fix asap. I'm ok to give you my blog login info if that helps. Do not reply by forum only by email so I can reply. Finally I'm confused about Mandrill. When would I need this? Autoresponder?
Thank you Kevin