There might be situations in which a reviewer cannot complete their assignments. If your reviewer group is setup to Automatically assign and is not setup to assign Every Application to Every Reviewer, the reviewer can be Locked. Locking a reviewer will remove any unsubmitted reviews (including started but not completed saved reviews) from the reviewer, and it will reassign those reviews to other members of the same group. The system will distribute this work evenly across the remaining reviewers. Any reviews already submitted by the reviewer will remain with the reviewer, and no new applications will be assigned to the reviewer.
Follow these steps in order to Lock a reviewer:
1) Select Opportunity > Evaluators > Groups
2) Select Reviewer Group where the reviews need to be redistributed
3) Select the Reviewers tab
4) Mark the Locked? checkbox next to the reviewer that is unable to complete their reviews
5) Select Update Reviewers
If you are unable to see the Locked? feature, you may have specific settings in place that are preventing the system from locking.