RadGrid does not show or isn't visible on Page_Load?
Set a default value for your datasource SelectCommand of the SqlDataSource of your Grid.
<asp:ControlParameter Name="FleetNo" ControlID="txtFleetNo" PropertyName="Text" DefaultValue="0"/>
I discovered this error, when using a MaintainFleet page and and an AddFleet page.
The maintain page was working perfectly with the RadGrid. So I copied the code over for my new Add page.
I changed my dropdown from the Maintain Page to a textbox on the Add page. That field is the control parameter of the Select statement . And unless you set a default value for the Textbox one, the grid will not be visible until a postback is done. Of course the code was working on the Maintain page.... the Dropdown had been set with a default value in the code.