If you are encountering issues with importing to certain fields, it is recommended that you create an import template.
To make sure your field names are correct for an import, start by exporting a small list from your lead database.
You can selected your required fields via the column chooser by right clicking a field in your list.
Then select Export list under List Actions, then select Visible columns or All columns in your export. In the exported .csv file check the first row; your correctly named fields should be listed in there. You can now copy the field names to your import list, or copy the cells from your list to this exported .csv. You should now be able to import without issue.
Please note: if you select All columns then the exported .csv file contains system fields that cannot be re-imported to. Please see the related article: Ignored Fields and Blocked Field Updates During List Import