When a proof has been locked for review by an approver, 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 in the bottom right hand corner of the proofing screen.
To request the proof be unlocked
Open the proof and click or hover over the lock icon, then select request unlock.
Those who are able to unlock the proof will be notified that you have asked them to unlock the proof. If they do so, you will receive an email and in-app notification that you can add comments to the proof.