How to re-edit post if my post was is edited by mod or admin

Hi, I am the admin of my forum, when I edit one post of my member, that member can no longer be able to edit his post again ! How to correct that? And how have have one specific group who can reedit the post even after admin edit your post before

Is the above setting enabled in your instance?

Thanks Bhanu Sharma, your solution is good, I have another question, is it possible manually lock or unlock a post

This came up in another topic recently: