As of v2 build 22 when a group contains a group item with a comma in it the comma acts as a delimiter and the whole item is not sent to MailChimp. For example: If the group item is "Cats, Dogs and Birds" then when you export and select this group item only the word "Cats" will be sent to MailChimp. As "Cats" is not a valid entry no records will be updated. What is more general i.e. non-email specific errors are not reported on by Chimpegration so no error is reported. This has been updated in the code and will be released in a future version but the underlying problem of the comma acting as a delimiter needs to be addressed.
As a workaround you should remove any commas from group items.