1. Ensure that there are no empty rows - To get a successful import, you should make the spreadsheet look as clean as possible, with a single header row on row 1, and the contact data starting on row 2. Any full blank row in your spreadsheet will be perceived as the end of the spreadsheet and any remaining information will not be imported.
2. Make sure that document is saved as an .xls or .xlsx file. This is a requirement in order to upload.
3. Check your spreadsheet for errors. It's possible there are unverifiable fields within the spreadsheet you've created. To do so, navigate to Formulas | Error Checking (see below) to correct any cells that may contain an error. When complete, you'll want to run that check 1 last time to ensure there are no remaining errors prior to re-saving the document.