In your aspx file make sure you have all the columns within grdview.
<columns>
<asp:boundfield headertext="PRODUCT_GROUP_ID" datafield="PRODUCT_GROUP_ID" sortexpression="PRODUCT_GROUP_ID" xmlns:asp="#unknown">
/>
<asp:boundfield headertext="PRODUCT_GROUP_DESC" datafield="PRODUCT_GROUP_DESC" sortexpression="PRODUCT_GROUP_DESC" />
</asp:boundfield></columns>
Also make sure you have not set Visible="false" for any of those columns you wanted to display.