This can be caused by the following reasons:

  1. The Do Not Synchronize field is marked on one or both of the contacts.
  2. One or both of the contacts in LCRM are tied to a LO constituent record that has been removed.
To resolve this:
  1. Uncheck the Do Not Synchronize field from the LCRM contact that will be the master record.
  2. Contact support to have the removed LO constituent reactivated.