When a proof has been locked for review by an approver or gatekeeper, the red pen is no longer available to other reviewers on the proof. A locked proof will display a lock icon on the proof tile, and also in the bottom right hand corner of the proofing screen.
To unlock the proof
Open the proof and click or hover on the lock icon, and select unlock proof.
Those who are also able to unlock the proof will be notified by email and in-app that the proof is now unlocked and proofing can continue.