"Provider=Microsoft.Jet.Oledb.4.0;Data Source=D:\\test\\aa.xls;Extended Properties='Excel 8.0;HDR=Yes;IMEX=1';" 
最后两个参数去掉试试!

------解决方案--------------------
还有一种情况!就是同一列上!如果类型不一致的话有时候也会出现漏掉的现象!

------解决方案--------------------
给你个例子 这是我以前已经测试好的
public void OpenExcel()
       {
           Application app = new Application();//Open Excel Application.
           Workbooks wbs = app.Workbooks;
           _Workbook wb = wbs.Add(@"D:\work\30yr_pmmsmnth.xls");//Open an existing workbook
           Sheets shs = wb.Sheets;
           _Worksheet wsh = (_Worksheet)shs.get_Item(1);//Select the first Sheet page

           int a = 8;
           int b = 20;
           for (int i = 0; i < 6; i++)
           {
               if (i == 0)
               {
                   for (; a < b; a++)
                   {
                       Range rr = wsh.get_Range("A" + a, "U" + a);
                       Object[,] saRet = (System.Object[,])rr.get_Value(System.Reflection.Missing.Value);
                       for (int n = 0; n < 7; n++)
                       {
                           string indexs = "";
                           if (n == 0)
                           {
                               indexs = "B5";
                           }
                           else if (n == 1)
                           {
                               indexs = "E5";
                           }
                           else if (n == 2)
                           {
                               indexs = "H5";
                           }
                           else if (n == 3)
                           {
                               indexs = "K5";
                           }
                           else if (n == 4)
                           {
                               indexs = "N5";
                           }
                           else if (n == 5)
                           {
                               indexs = "Q5";
                           }
                           else if (n == 6)
                           {
                               indexs = "T5";
                           }
                           Range range0 = wsh.get_Range(indexs, System.Reflection.Missing.Value);
                           Pmmsmnthyr pp = new Pmmsmnthyr();
                           pp.WeekDay = saRet[1, 1].ToString() + "/" + range0.Value2.ToString();
                           pp.Rate = saRet[1, 2 + n * 3].ToString();
                           pp.Pts = saRet[1, 3 + n * 3].ToString();
                       }
                   }
               }
}

------解决方案--------------------
 给你关键代码，这段代码我使用了好几年了
fileName = this.txtexcel.Text.Trim();
           if (System.IO.File.Exists(fileName) == false)
           {
               MessageBox.Show("請確認Excel檔的位置,路徑請設置為英文或數字,並盡可能短,謝謝!");
               return;
           }
GetWindowThreadProcessId(t,out k);
			System.Diagnostics.Process pe=System.Diagnostics.Process.GetProcessById(k);  
			object missing=Type.Missing;