The reason may be due to missmatching information contained in the user import file or that the user simply does not exist in the file.
Resolve this by editing the information contained in the portal to match the user import file.
If the user is not at all in the user import file, then it must be added, make sure that the information complies with the information in the portal.
If you already have all information in your user import file and can not find the user, you may need to enable the user. Follow this LINK to solve this!