When a proof has been locked for review by an approver, the red pen is no longer available to others 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’s info pane either by clicking the info icon on the proof tile, or when you’re viewing the proof.
Click yes, unlock.
Those who are able to unlock the proof will be notified by email and in-app that the proof is now unlocked and proofing can continue.