Approval Serial Heirarchy

Content

We have defined approval hierarchy in 'Approval Units' form. We have defined an Owner (person A) and 2 Reviewers (person B & person C). 

Our requirement is to create a serial approval hierarchy. Eg; Once owner (person A) promote the task approval should goes to the first approver (person and when he approves it approval should goes to the next person (person .

But my issue is, our approval process stops at Person B. Which means once the 'person A' promotes it approval goes to 'person B' and it completes the process. It does not goes to 'Person C'

How to sort this issue??