Mailchimp Sync plugin not working correctly

I have the MailChimp Sync plugin running and I thought it had been working properly, but I was noticing today that we have 313 registered users in Wordpress but only 258 in Mailchimp. I deleted some users in Wordpress but they aren't getting removed in the MailChimp list. I ran the Import and it said there were 313 errors out of 313 users. I downloaded one of the JSON logs and it looks like it's returning a 400 error on every request.