If you've assigned Reviewers to specific Idea Groups and have confirmed there are ideas in those groups to be reviewed, but there are no ideas awaiting review in individual work lists, then check the setup of the Idea Group name. If there is any spacing in the group name, then retype the name using only the keyboard space bar to add desired spaces. If names are copied and pasted from an outside application, invisible characters are often added that may adversely affect behavior in Innovation Central.
