Gridview Paging checkbox not assinging properly

I have data in my gridview and a pagesize = 2. Also each row in gridview has a checbox for approve/reject.

So If I checked 2 rows from 1st page and 2 rows from 2nd page, it only approves the last two page checked from the 2nd page. Why this happens I dont know. I want all the 4 records to get approved.

Below is my code which I wrote,

ASPX

<asp:GridView ID="grdDisplayCMMData" runat="server" AutoGenerateColumns="false" Width="100%" ShowHeaderWhenEmpty="true" CssClass="heavyTable table" EmptyDataText="No records to display"
            AllowPaging="true" PageSize="2" OnPageIndexChanging="grdDisplayCMMData_PageIndexChanging">
            <Columns>                    
                <asp:TemplateField HeaderText="ID" Visible="false">
                    <ItemTemplate>
                        <asp:Label ID="lblID_CMM" runat="server" Text='<%#Eval("ID") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="SAP ID">
                    <ItemTemplate>
                        <asp:Label ID="lblSAP_ID_CMM" runat="server" Text='<%#Eval("SAP_ID") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="ID OD CHANGE">
                    <ItemTemplate>
                        <asp:Label ID="lblID_OD_COUNTCHANGE_CMM" runat="server" Text='<%#Eval("ID_OD_COUNTCHANGE") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="ID OD CHANGE DATE">
                    <ItemTemplate>
                        <asp:Label ID="lblID_OD_CHANGEDDATE_CMM" runat="server" Text='<%#Eval("ID_OD_CHANGEDDATE") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="RRH COUNT CHANGE">
                    <ItemTemplate>
                        <asp:Label ID="lblRRH_COUNTCHANGE_CMM" runat="server" Text='<%#Eval("RRH_COUNTCHANGE") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="RRH COUNT CHANGE DATE">
                    <ItemTemplate>
                        <asp:Label ID="lblRRH_CHANGEDDATE_CMM" runat="server" Text='<%#Eval("RRH_CHANGEDDATE") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="TENANCY COUNT CHANGE">
                    <ItemTemplate>
                        <asp:Label ID="lblTENANCY_COUNTCHANGE_CMM" runat="server" Text='<%#Eval("TENANCY_COUNTCHANGE") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="TENANCY COUNT CHANGE DATE">
                    <ItemTemplate>
                        <asp:Label ID="lblTENANCY_CHANGEDDATE_CMM" runat="server" Text='<%#Eval("TENANCY_CHANGEDDATE") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="STATUS">
                    <ItemTemplate>
                        <asp:Label ID="lblSTATUS_CMM" runat="server" Text='<%#Eval("STATUS") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="CREATED BY" Visible="false">
                    <ItemTemplate>
                        <asp:Label ID="lblCREATEDBY_CMM" runat="server" Text='<%#Eval("CREATED_BY") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Approve/Reject">
                    <ItemTemplate>
                        <asp:CheckBox ID="chkApprRejCMM" runat="server" />
                    </ItemTemplate>
                </asp:TemplateField>
            </Columns>                
        </asp:GridView>

C# CODE

protected void btnApproveCMM_Click(object sender, EventArgs e)
    {
        try
        {
            bool flgCMM = false;
            IPColoFields ObjIPColoFields = new App_Code.IPColoFields();
            List<IPColoBilling_BKP.App_Code.UMS.UMSGroupDetails> UMSGroupDetails = (List<IPColoBilling_BKP.App_Code.UMS.UMSGroupDetails>)Session["lstUMSGroupDetails"];

            Session["lstUMSGroupDetails"] = UMSGroupDetails;
            string strApprove = "";


            foreach (GridViewRow gvrow in grdDisplayCMMData.Rows)
            {
                var checkbox = gvrow.FindControl("chkApprRejCMM") as CheckBox;

                if (checkbox.Checked)
                {
                    flgCMM = true;

                    int Id = Convert.ToInt32((grdDisplayCMMData.Rows[gvrow.RowIndex].FindControl("lblID_CMM") as Label).Text);

                    ObjIPColoFields.Unique_Id = Id;
                    ObjIPColoFields.UMS_GRP_BY_ID = intCurrentGrpId;
                    ObjIPColoFields.UMS_GRP_BY_NAME = strCurrentGrp;
                    ObjIPColoFields.UMS_GRP_TO_ID = UMSGroupDetails[1].GroupID;
                    ObjIPColoFields.UMS_GRP_TO_NAME = UMSGroupDetails[1].GroupName;
                    ObjIPColoFields.FCA_STATUS = "1";
                    ObjIPColoFields.LAST_UPDATED_BY = lblUserName.Text;
                    strApprove = CommonDB.Approve_IPCOLO_CMMLevel(ObjIPColoFields);
                }
            }

            BindCMMData();
            if (flgCMM == false)
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('Please check atleast one row'); window.location ='IpColoDefault.aspx';", true);
            }
            else
            {
                ScriptManager.RegisterStartupScript(this, this.GetType(), "alert", "alert('Record Approved successfully'); window.location ='IpColoDefault.aspx';", true);

            }

        }
        catch (Exception ex)
        {
            string strErrorMsg = ex.Message.ToString() + " " + "StackTrace :" + ex.StackTrace.ToString();
            CommonDB.WriteLog("ERROR:" + strErrorMsg, ConfigurationManager.AppSettings["IPCOLO_LOG"].ToString());
        }
    }

Also see the paging code

protected void grdDisplayCMMData_PageIndexChanging(object sender, GridViewPageEventArgs e)
    {
        try
        {
            BindCMMData();
            grdDisplayCMMData.PageIndex = e.NewPageIndex;
            grdDisplayCMMData.DataBind();
        }
        catch (Exception ex)
        {
            string strErrorMsg = ex.Message.ToString() + " " + "StackTrace :" + ex.StackTrace.ToString();
            CommonDB.WriteLog("ERROR:" + strErrorMsg, ConfigurationManager.AppSettings["IPCOLO_LOG"].ToString());
        }
    }