When creating a CSV import file make sure the zip / postal code column is properly formatted as text so you do not lose leading zeros.
The column header for postal/zip codes is zip or sl_zip.
In the United States many northeastern postal codes start with zero. Many sheets, Microsoft Excel in particular, LOVE to lead off the leading zero if you enter a zip code as 03907 and save as CSV.
In Excel, and many other apps, you can force a field where you are typing in numbers to be text by starting your entry with an apostrophe (‘). Type ‘03907 when entering the data NOT 03907.
A good test is to open your CSV file in Google Sheets and see what it looks like. Does it look like the sheet above missing a zero? The data is likely wrong.
Another indicator that this column is set to a numeric type and not an text type is the data is on the RIGHT SIDE of the cell. Text is left-justified (like sl_state) and numbers are right-justified.
If you are using the WPSLP “do it yourself” plugin with Power add-on and you have not included the latitude and longitude and require geocoding , make sure you have a Google API account and have checked off the Maps and Geocoding APIs in your library.