To make sure the Default Contact is set up correctly, please navigate to the following to check the Contact record:
Go into Synergy.
Click SEN.
Click Admin, Contacts, then Contacts.
Then search for the contact.
The tick box should be selected for A Default Contact.
If this is selected, please attempt to Batch Update Child Contacts by going into:Click Admin.
Click Contacts.
Select Batch Update Child Contacts.
From here, if you still cannot see the Contact record displayed within the Contacts tab of the Child Record, then go back into the record and check to see if there is an End Date there. If there is, check to see if the End Date is today or before that. Remove the End Date or push it forward to a future End Date, and then save and Batch Update again.