You are using Case Merge Premium and notice that emails added to a Child Case post-merge aren't being rolled up to the associated Parent Case.
Each time an email is added to a case, Case Merge Premium checks the following items to determine if an email roll-up is necessary.
1. Does the status of the case where the email is being added match the Merged Case Status in the Case Merge Premium settings?
2. Is the 'Parent Case' field on the child case populated?
If the answer to both items is Yes, than emails added to the case you're working with should roll-up properly to the respective Parent Case. If both of these items are properly configured and you're continuing to experience an issue with emails rolling up, please open a support case with us by sending an email to firstname.lastname@example.org.