想要每个表生成一个图表，遍历各个表的时候失败，只有第一个成功怎么办

Dim a As Integer = CInt(e.Form.Controls("TextBox1").text)

Dim app As New MSExcel.Application()

app.Visible = True

Dim book As MSExcel.Workbook = app.Workbooks.Open("D:\\临沂分段.xls")      \'指定excel文件

For j As Integer = a To 1 Step -1

    Dim ws As MSExcel.Worksheet = book.Sheets("临沂第" & j & "段")      \'指定工作表

    \'Dim ws As MSExcel.Worksheet = book.Sheets(j)

    If ws.ChartObjects.count > 0 Then

        ws.ChartObjects.Delete \'删除工作表上已经存在的图表

    End If

    Dim Rg1 As MSExcel.Range = ws.Range("C3:C120")

    Dim Rg2 As MSExcel.Range = ws.Range("D1:E120")

    Dim Co As MSExcel.ChartObject = ws.ChartObjects.Add(100,30,400,250)   \'创建图表对象

    Dim Cht As MSExcel.Chart = Co.Chart

    Dim s As MSExcel.Series

    With Cht

        .ChartType = MSExcel.XlChartType.xlLine

        .SetSourceData(Rg2)

        s = Cht.SeriesCollection(1)

        s.XValues = Rg1

    End With

    With Cht.Axes(MSExcel.XlAxisType.xlCategory)   \'获取X轴

        .HasTitle = True

        .AxisTitle.Text = "手部距离"   \'添加标题

        .HasMajorGridlines = True  \'显示主要网格线

        \'.HasMinorGridlines = True  \'显示次要网格线

        .MinimumScale = 100

        .MaximumScale = 500

        .MinorUnit = 1

        .MajorUnit = 10

        .CrossesAt = 400

        .ReversePlotOrder = False

    End With

    With Cht.Axes(MSExcel.XlAxisType.xlValue) \'获取Y轴

        .HasTitle = True

        .AxisTitle.Text = "埋深(m)"   \'添加标题

        .HasMajorGridlines = True\'显示只要网格线

        .HasMinorGridlines = True\'显示次要网格线

    End With

Next j

必须先在表中插入列