Due to limitation in Constant Contact API that does not return detailed error 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 have been unsubscribed previously so cannot be updated by SyncApps.