Within MailChimp for individual lists you have the option to make certain Fields required. This means you can only create new/update existing subscribers in MailChimp if values have been entered for these fields within Workbooks. (Click to enlarge image.)

If you are using the MailChimp integration then you can synchronise Workbooks Marketing Campaigns and their Members across to MailChimp. However, if you have made any fields 'Required' within MailChimp then you need to make sure all the records you synchronise across from Workbooks contain values for these fields. If any fields required by MailChimp are left blank MailChimp will reject the Person and they won't be synchronised into MailChimp.

If this is the case you could either mark the Fields within MailChimp as not required or you could ensure that you capture this information within Workbooks, possibly by making the same fields required.