FindControl not finding controls

I have a form using a master page that has many dynamically created controls each have a unique id assigned and nested at verios levels within other controls and I'm trying to access them from the code behind but the find control method is not finding them.  The parameter is the unique id of a textbox.  All unique ids follow the same pattern so I can readily identify associated objects. I have verified that the ids are correct and that they do exist when I view the source. Any ideas?
Dim content As ContentPlaceHolder = CType(Page.Master.FindControl("ContentPlaceHolder1"), ContentPlaceHolder) Dim img1 As Image = CType(content.FindControl(e.Parameter.Replace("|hrs|", "|img1|")), Image) Dim img2 As Image = CType(content.FindControl(e.Parameter.Replace("|hrs|", "|img2|")), Image) Dim img3 As Image = CType(content.FindControl(e.Parameter.Replace("|hrs|", " ...