Filtering by completed assessments only will show you only those applicants who have completed all assessments they were invited to take. For example, if the job requires two assessments and the applicant has only completed one, this applicant will not be displayed when the Completed Assessments filter is applied.
To go back to viewing all applicants regardless of whether they have completed assessments, check the box next to Any Assessment Status or simply Reset All Filters with the button located on the right-hand side of your screen.
Another way to show only applicants who have completed assessments is by looking at the listings on your Jobs tab. Under each hiring step are the names of applicants who are currently in the step. Any of the steps to the right of the Assessing step will show applicants who have completed their assessments.