Please let us know whether the parent request is in the status of Open or Closed. If it is in the status of Closed we request you to check the option under Admin-> Self Service Portal Settings-> Request Feature List, to configure the action to be performed when the requester replies through E-mail to the closed requests.
If this is not the case please send in your contact details to Support@servicedeskplus.com by referring this forum post so that we can have a remote session to look into this issue.
Thanks,