When you're nesting master pages, you'll get an extra container "Content" you need to look through.
As a result, if you're trying to use FindControl from a given child page the usual approach is something to the effect of:
Label myLabel = (Label)this.Master.FindControl("myLabel");myLabel.Text = "Success!";Since we have a nested master page, with "myLabel" in the child master, this control will be contained within a content control.
So, this changes the code to:
ContentPlaceHolder ph = (ContentPlaceHolder)this.Master.Master.FindControl("yourContentPane");Label myLabel = (Label)ph.FindControl("myLabel");myLabel.Text = "Success!";and in VB.NET
Dim ph As ContentPlaceHolder = DirectCast(Me.Master.Master.FindControl("yourContentPane"), ContentPlaceHolder)Dim myLabel As Label = DirectCast(ph.FindControl("myLabel"), Label)myLabel.Text = "Success!"The content from the child page is loaded into the first master page control, which is subsequently loaded into the grandparent master page.