If your spreadsheet contains one or more missing product IDs the row(s) containing the missing product ID(s) will be skipped and those product(s) will not be imported.
To correct this add product IDs to those cells and try the import again.
How to find and highlight blank cells in Excel 2016 and 2013
1. Select your data.
2. Go to Home > Conditional Formatting > New Rule
3. Select Format only cells that contain and set Format only cells with to Blanks and click OK.