I had a look in my database at the field names in the user and user meta tables and made sure that the column headers in the excel-imported csv matched them. The plugin I’d used to export the users had altered some of these.
I still had no joy so then I deleted the first column in the excel-imported csv (a column of ids) on the basis that the new db would be assigning its own (new) ids to the users.
It worked. n.b the csv created has to be saved as “Windows csv”.
Incidentally, the plugin I used for exporting the original users is “Users to CSV”, which was the only one I found that exported absolutely everything, which for me was necessary as my users can also login to the site with Facebook, and most of the export plugins only export the basic user data.
Hope this helps.