Version : 3.0.2-20080204

In HSSFWorkbook the getSheet function is case sensitive :
"
  public HSSFSheet getSheet(String name)
    {
        HSSFSheet retval = null;

        for (int k = 0; k < sheets.size(); k++)
        {
            String sheetname = workbook.getSheetName(k);

            if (sheetname.equals(name)) // here is the problem
            {
                retval = (HSSFSheet) sheets.get(k);
            }
        }
        return retval;
    }
"

MS Excel or OpenOffice are non case sensitive for sheetName : equalsignorecase should be used instead of equals.