If one or more of your unique identifiers (product ID, SKU, Model number, ISBN, etc.) are duplicated the first product with the identifier will be imported but the second will not. For example, in the image below the SKU number "DH-00003" is duplicated. In this case, the Bluetooth Speaker will be imported but not the Portable DVD Player.
If this happens check for duplicates in your spreadsheet. You can easily do this using Excel. (The instructions here are for Excel 2016 and 2013.)
How to find duplicates in your spreadsheet
1. Select the range of cells that you believe might contain duplicates.
2. From Excel's Home tab choose Conditional Formatting > Highlight Cells Rules > Duplicate Values:
3. Choose a formatting for your duplicate values and click OK.
4. Any duplicate values are now identified.