How to keep layout style of menu in a custom component

Hi All,

I am struggling to find a way to keep layout of menu (actually a sub menu) in a custom component. As shown in the image, the sub menu loose the nice layout, especially the column icons and tick mark is displayed. Could any one help me with this? Code is attached. A ToolStripMenuItem is added to main menu and its DropDown property is assigned to a new ToolStropDropDown. 

Thank you.

Xiaoming

public override void AppendAdditionalMenuItems(ToolStripDropDown menu)
{

Menu_AppendSeparator(menu);

//add view option
ToolStripMenuItem previewDropDownMenuItem = new ToolStripMenuItem();
previewDropDownMenuItem.Text = "Preview Option";

ToolStripDropDown previewOptionsDropDown = new ToolStripDropDown();

previewIES = new ToolStripMenuItem();
previewIES.Text = "Candela Profile";//Preview with candela profile
previewIES.Checked = true;//use check display for this sub men
if (displayMethod==0)
previewIES.CheckState = System.Windows.Forms.CheckState.Checked;
else
previewIES.CheckState = System.Windows.Forms.CheckState.Unchecked;
previewIES.Click += new EventHandler(displayMethod_Changed);
previewOptionsDropDown.Items.Add(previewIES);

previewCone = new ToolStripMenuItem();
previewCone.Text = "Half Power Cone";//
previewCone.Checked = true;//use check display for this sub men
if (displayMethod == 0)
previewCone.CheckState = System.Windows.Forms.CheckState.Unchecked;
else
previewCone.CheckState = System.Windows.Forms.CheckState.Checked;

previewCone.Click += new EventHandler(displayMethod_Changed);
previewOptionsDropDown.Items.Add(previewCone);

previewOptionsDropDown.LayoutStyle = menu.LayoutStyle;
previewDropDownMenuItem.DropDown = previewOptionsDropDown;
menu.Items.Add(previewDropDownMenuItem);

GH_DocumentObject.Menu_AppendTextItem(previewOptionsDropDown, iesDisplaySize.ToString(), new GH_MenuTextBox.KeyDownEventHandler(PreviewSize_keydown), new GH_MenuTextBox.TextChangedEventHandler(PreviewSize_TextChange), true);

GH_DocumentObject.Menu_AppendItem(menu, "Load IES", new EventHandler(this.Menu_LoadIESClicked), true);//load IES file
Menu_AppendSeparator(menu);

}