Problem TreeView OnTreeNodePopulate event

Question

Hi. I am using a treeview in asp.net 3.5(using c#.net),which is populated from sqldatabase. OnTreeNodePopulate event fires this event,

protected void tvLabs_TreeNodePopulate(object sender, System.Web.UI.WebControls.TreeNodeEventArgs e) { PopulateSubLevelLab(e.Node.Value, e.Node); //This function is used for populating the child node. FillLabGrid(e.Node.Value);---My Question? }

---My Question? The FillLabGrid method used for fill a grid view on the basis of selecting the treeview parent node. But this function filled the grid only once when i click the parent node for the first time. Secondly when i collapse the treeview and again click on the parent node function (tvLabs_TreeNodePopulate)is not called?

Hope you are getting my point. Please advice me. Thanks in advance.