This might happen if those contacts have already been added to Zoho Campaigns as subscribers. Check if those contacts are already added. There can be many other reasons why some of the contacts were ignored. You can view the sync report in the detailed view of the sync.
In order to view the details of the sync created,
- Navigate to Contacts.
- Click Sync Services and click Active Sync under Zoho Books.
- Click View details of the particular sync.