Regularly when importing large lists of supporters into your people dashboard, you will see a column for errors, with a number and a linked CSV file. NationBuilder strives to keep your imports clean and free of bad data, so ususally these errors have to do with something like typos in email addresses in your rows. You can download the CSV file and in the last column it will show what error prevented those rows from being imported. You can then clean up the data and reimport those records if you like.