A Constituent will not sync with the Raiser's Edge Connector if they are marked in Resolve New Registrations. You can check this by doing the following:
  1. Go to Data Management > select Duplicate Management.
  2. Click the Resolve New Registrations tab.
  3. Search for the Constituent that is not syncing.
  4. If found, determine if the record is a duplicate and click Not a Match, Match, or Merge.