By default, the list shown on the panel within suppression feature belongs to all the email ids which were added to the suppression list within the last 30 days. There is a high possibility that the user could have added to the suppression list before the last 30 days and hence you are not able to search him with the default filters available on the suppression page.

In order to fix this, click on the date filter as given in the below image and enhance your search by increasing the date range so as to get accurate results when searching the suppressed email ids.