Dropdownlist with Modal

Question

good afternoon. I have a screen with a grid made in webforms, and when clicking the existing button on the grid, a modal is displayed with the information of the corresponding record. In this modal I have some dropdownlists linked to tables. Only that I put the event Onselectedindexchanged no longer opens the modal. Has anyone ever been through this problem? Modal call:

System.Text.StringBuilder sb = new System.Text.StringBuilder();
            sb.Append(@"<script type='text/javascript'>");
            sb.Append("$('#editModal').modal('show');");
            sb.Append(@"</script>");
            ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "EditModalScript", sb.ToString(), false);

Here the dropdownlist creation code:

<asp:DropDownList ID="DDLEditReq" runat="server" DataSourceID="SQLEditRec" style="width:340px"
                                            DataTextField="nome_usuario" DataValueField="id_usuario" ValidationGroup="DDL" AutoPostBack="True"
                                            OnSelectedIndexChanged="ddlb_req"></asp:DropDownList>

And here the code of the creation of the modal:

 <div id="editModal"  tabindex="-1" class="modal hide fade" role="dialog" aria-labelledby="editModalLabel" aria-hidden="true">