A reason for not being able to find a user can depend on the status of that user being set to inactive, lack of permission to view the user or the user is missing in the user import file.
To figure out if you have the permission to view all the users, compare your list count with an administrator with the highest authority. If not, consult the support for further help.
To search for a "hidden" user in the portal follow the steps below.
Navigate to Administrator -> Users
Enter the name in the search field and klick on search to find the user, remove the "Active" filter to view inactive users.
Find the name in the search result and klick on the name.
*If for some reason it does not show a result, you have to look into the user import file and possibly add that user.
Make the user Active and save the change.
If the user keeps getting inactivated but should not be, it may be due to the automatic user import, follow this GUIDE to solve this issue.
It is important that allt the fields match with the information you have in your user import file, otherwise the user will reset to inactive on the next automatic user import.