Due to a limitation in Constant Contact API that does not return detailed errors SyncApps cannot report which email addresses have errors. Only error total is displayed like in the following text:
[Processed records=7, Updated=5, Error=2, Flagged as Opted-Out=0, Duplicated Email Address=0, Records With Blank Email Address=0, Previously Bounced=0, Filtered=0]
To see the detailed error you can open Constant Contact Activity Reports. Please click Contacts > Activity menu.
Usually, the errors are caused by contacts who have been unsubscribed previously so cannot be updated by SyncApps.