RadGridView - CustomGrouping event is not firing when you change some cell value and RadGridView is data bound via DataSource property.

Workaround:
1.Subscribe to CellEndEdit event of RadGridView:
            this.radGridView1.CellEndEdit += new GridViewCellEventHandler(radGridView1_CellEndEdit);

2. Call Refresh method of MasterTemplate:

        void radGridView1_CellEndEdit(object sender, GridViewCellEventArgs e)
        {
            this.radGridView1.MasterTemplate.Refresh();
        }