When using the Data Import Wizard with the NPSP Data Importer and noticing that the Mailing Street address field was missing on all records after import, one likely cause is incomplete data in the CSV file. Here’s a detailed explanation:

Check the CSV File:

Open the CSV file that was uploaded.

Verify if the Mailing Street address column contains complete and properly formatted data for all rows.

Ensure Completeness:

Ensure that there are no missing values or improperly formatted addresses in the Mailing Street column.

Even a single missing value can cause the entire column to be skipped during import.

Re-import Data:

Correct any incomplete or missing data in the CSV file.

Use the Data Import Wizard again to re-import the corrected file.

Verify Field Mappings:

During the import process, ensure that the Mailing Street field is correctly mapped to the appropriate Salesforce field.

By ensuring the completeness and correctness of the data in the CSV file, you can avoid issues where fields are missing after import.

Salesforce Data Import Wizard Guide: Data Import Wizard.