Setting SqlDatasource Parameters From TextBox

I have a sqlDatasource with 3 parameters based on the input of 3 text boxes on the page. The datasource returns sales details for a company based on a from and to date. I am attempting to set the value of the 3 parameters in the Selecting event of the datasource control but I'm not getting any data back. If I set the values literally then I get data back. Also when I step through the code I can see the 3 parameters getting their values from the textboxes and the drop down list as they should. This is driving me insane as I'm new to .net and just can't see what is stopping me retrieving the data when using the form fields to set the datasources parameters. Below is the aspx and the code behind for the page. Thanks in advance for any help.
 aspx.....
 
<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="LabelSales.aspx.cs" Inherits="LabelSales" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
   
    <asp:SqlDataSource ID="SqlDataSourceSales" runat="server" EnableCaching="false"
        ConnectionString="<%$ ConnectionStrings:streetwisedigitalConnectionString %>"
        SelectCommand="DL_GET_SALES_BY_LABEL" SelectCommandType="StoredProcedure" OnSelecting="SqlDataSourceSales_Selecting" >
        <SelectParameters>
            <asp:Parameter Name="fromDate" />
            <asp:Parameter Name="toDate" />
            <asp:Parameter Name="label" />
        </SelectParameters>
    </asp:SqlDataSource>
   
    <asp:SqlDataSource ID="SqlDataSourceLabels" runat="server"
        SelectCommand="select label_id, label_name from dl_label order by label_name asc"
        ConnectionString="<%$ConnectionStrings:streetwisedigitalConnectionString%>">
    </asp:SqlDataSource>
    <div>
        <table>
            <tr>
                <td>Start Date:&nbsp;</td>
                <td>
                    <asp:TextBox ID="FromDate" Runat="server" Width="70"/>
                    <asp:Button ID="btnFrom" Runat="server" Text="..." UseSubmitBehavior="false" />
                </td>
            </tr>
            <tr>
                <td>End Date:&nbsp;</td>
                <td>
                    <asp:TextBox id="ToDate" Runat="server" Width="70" />
                    <asp:Button ID="btnTo" Runat="server" Text="..." UseSubmitBehavior="false" />
                </td>
            </tr>
            <tr>
                <td>Label:&nbsp;</td>
                <td>
                    <asp:DropDownList ID="LabelList" Runat="server"
                    DataSourceID="SqlDataSourceLabels"
                    DataTextField="label_name" DataValueField="label_name">
                    </asp:DropDownList>
                </td>
            </tr>
            <tr>
                <td colspan="2" align="right">
                    <asp:Button ID="Button1" OnClick="SubmitButton_OnClick" Text="View Sales" runat="server" />
                </td>
            </tr>
        </table>
        <p>
            <asp:GridView ID="GridViewSales" runat="server"
                DataSourceID="SqlDataSourceSales"
                ShowFooter="True"
                AllowSorting="True"
                AutoGenerateColumns="False"
                OnRowDataBound="GridViewSales_RowDataBound" EmptyDataText="No data to display.">
                <Columns>
                    <asp:BoundField DataField="cat_no" HeaderText="Cat No" />
                    <asp:BoundField DataField="artist" HeaderText="Artist" />
                    <asp:BoundField DataField="title" HeaderText="Title" />
                    <asp:BoundField DataField="remix" HeaderText="Remix" />
                    <asp:BoundField DataField="qty" HeaderText="Sold" />
                    <asp:boundfield datafield="commission"
                        HtmlEncode="False"
                        dataformatstring="{0:F2}" 
                        headertext="Commission">
                        <ItemStyle HorizontalAlign="Right" />
                    </asp:boundfield>
                </Columns>
            </asp:GridView>
        </p>
    </div>
</asp:Content>

code behind...
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class LabelSales : System.Web.UI.Page
{
    int totalSold;
    decimal totalCommssion;

    protected void GridViewSales_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        // add column totals to gridview
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            totalSold += (int)DataBinder.Eval(e.Row.DataItem, "qty");
            totalCommssion += (decimal)DataBinder.Eval(e.Row.DataItem, "commission");
        }
        // display the totals
        else if (e.Row.RowType == DataControlRowType.Footer)
        {
            e.Row.Cells[0].Text = "<b>Total</b>";
            e.Row.Cells[4].Text = totalSold.ToString();
            e.Row.Cells[5].Text = totalCommssion.ToString("f2");
        }
    }
   
    protected void Page_Load(object sender, EventArgs e)
    {
              
    }
    protected void SqlDataSourceSales_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
    {
        // *** This method does not work ***
        e.Command.Parameters[0].Value = FromDate.Text;
        e.Command.Parameters[1].Value = ToDate.Text;
        e.Command.Parameters[2].Value = LabelList.Text;

        // *** This method works! ***
        e.Command.Parameters[0].Value = "02/01/2007";
        e.Command.Parameters[1].Value = "02/01/2008";
        e.Command.Parameters[2].Value = "Fat!";
    }

    protected void SubmitButton_OnClick(object sender, EventArgs e)
    {
        SqlDataSourceSales.Select(DataSourceSelectArguments.Empty);
    }
}
 
 Many thanks
 
Simon