nested gridview text box value not getting in page index changing

<asp:GridView ID="gvSections" runat="server" AutoGenerateColumns="false" CssClass="Grid" DataKeyNames="section_id" Width="80%" Style="padding-left: 100px">
<asp:TemplateField ItemStyle-VerticalAlign="Top" ItemStyle-HorizontalAlign="Justify">
<asp:ImageButton ID="imgShow" runat="server" OnClick="Show_Hide_ChildGrid" ImageUrl="~/Images/plusicon.png" CommandArgument="Show" />
<asp:Label ID="lblSectionDis" runat="server" Text='<%# Bind("section_name") %>' Font-Bold="true" Style="font-size: 16px;">
<asp:Panel ID="pnlOrders" runat="server" Visible="false" Style="position: relative">
<asp:GridView ID="gvQuestionAnswer" runat="server" AllowSorting="False" BorderWidth="2px" CellPadding="2" ForeColor="Black" GridLines="None" Width="100%" AutoGenerateColumns="false" OnRowDataBound="gvQuestionAnswer_RowDataBound" AllowPaging="True" OnPageIndexChanging="gvQuestionAnswer_PageIndexChanging" PageSize="5">
<asp:TemplateField HeaderStyle-Width="50" HeaderStyle-HorizontalAlign="Left" ItemStyle-HorizontalAlign="Center">
<%# Container.DataItemIndex+1 %>
<asp:TemplateField HeaderText="Question No" HeaderStyle-HorizontalAlign="Left" Visible="false">
<asp:Label ID="lblQuestionId" runat="server" Text='<%# Bind("question_id") %>' Width="150">
<asp:TemplateField HeaderStyle-HorizontalAlign="Left">
<asp:Label ID="lblSectionnew" runat="server" Text='<%# Bind("section_name") %>' Width="700" Visible="false">
<asp:Label ID="lblSectionId" runat="server" Text='<%# Bind("section_id") %>' Visible="false">
<asp:Label ID="lblQuestion" runat="server" Text='<%# Bind("qm_desc") %>'>
<asp:TextBox ID="txtAnswer" runat="server" TextMode="MultiLine" Width="450" Text='<%# Bind("Answer") %>'>
<asp:RequiredFieldValidator ID="RR1" runat="server" ControlToValidate="txtAnswer" Display="None" ValidationGroup="sub" ErrorMessage="Please Enter Answer" ForeColor="Red" SetFocusOnError="true">
<asp:ValidatorCalloutExtender ID="ValidatorCalloutExtender13" runat="server" Enabled="True" TargetControlID="RR1">
<asp:BoundField ItemStyle-Width="150px" DataField="section_id" HeaderText="Section Id" Visible="false" />
<asp:BoundField ItemStyle-Width="150px" DataField="section_name" HeaderText="Section Name" Visible="false" />

protected void gvQuestionAnswer_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        try
        {
            DataTable dtpageindex = new DataTable();
            dtpageindex = ViewState["SectionQuestions"] as DataTable;
            gvSections.PageIndex = e.NewPageIndex;
            string strSection; string strAnswer; string strQuestionid;
            foreach (GridViewRow row in gvSections.Rows)
            {
                if (row.RowType == DataControlRowType.DataRow)
                {

                    GridView gv = row.FindControl("gvQuestionAnswer") as GridView;
                    foreach (GridViewRow gvrow in gv.Rows)
                    {
                        if (gvrow.RowType == DataControlRowType.DataRow)
                        {

                            Label lblQuestionId = gvrow.FindControl("lblQuestionId") as Label;
                            Label lblSectionId = gvrow.FindControl("lblSectionId") as Label;

                            TextBox txtAnswer = gvrow.FindControl("txtAnswer") as TextBox;
                            strAnswer = txtAnswer.Text;
                            strQuestionid = lblQuestionId.Text;

                            foreach (DataRow dr in dtpageindex.Rows)
                            {
                                if (dr["question_id"].ToString() == strQuestionid)
                                {
                                    dr["Answer"] = strAnswer;
                                    dtpageindex.AcceptChanges();
                                }
                            }
                        }
                    }
                    ViewState["SectionQuestions"] = dtpageindex;
                    gv.PageIndex = e.NewPageIndex;
                    gv.DataSource = dtpageindex;
                    gv.DataBind();
                }
            }
        }
        catch (Exception ex)
        {}
    }