Approve/Reject Workflow - AEM 6.2

Hi,

I have created approve/reject workflow for my project. Below is the workflow steps

    Author would be initiating the workflow for a page and Reviewer group users will get notified in their AEM inbox for approve or reject.

    if One of the reviewer group user approves it, Final approver group users will get notified in inbox. If rejected, Author will be notified.

    If Any one of the approver group users approve the content, page will be activated. If rejected, reviewer group users will get notified.

I created these steps and reviewer and approver users and assigned to respective groups. When i test this scenario as admin login, it works without any errors. But if login as reviewer group, i am able to get the mail in inbox  but i am not able to proceed it to next level. Which means, if i click complete and select next step, I am getting "Could not complete workflow step".

What is the issue here? What am i missing?

Thanks in advance.

Divya