One thing that can cause this is where Archer is unable to locate the users when the users are part of a domain and you are passing the domain in your import. If you use the default import settings, the escape character will remove the backslash.
For example, you have a csv that specifies domain\user like the following:
In the Data Import Wizard, notice the backslashes are removed.
The default escape character is set to backslash so you will need to select a different escape sequence character when performing the import like a tilde (~). Making this change should allow the import to run successfully.