The most common reason for this is because the CSV file that is being used was originally generated as an Excel (XLS or XLSX) file.  Excel will place formatting code in some cells that is proprietary to Microsoft Excel.  This formatting is often not viewable as text and the the CSV file format will look at this formatting as text even though the cell is visually blank on screen.

To resolve this issue, complete the following steps:
 
  1. Open the original file in Microsoft Excel
  2. Select all cells that have content
  3. Copy the content ([CTRL +C] or right click and choose "copy")
  4. Open a brand new, blank Excel sheet
  5. Paste the content to the new sheet [(CTRL + P] or right click and choose "paste" in the upper left most cell.
  6. Save the file as a CSV file using a different name
  7. Re-upload the file to the imported list