How can I find a particular pushbutton?

On my form, which uses a master page and dynamically-loaded user controls, I have a pushbutton named btnOvpymtNotes for displaying a modal popup box to review notes that have been entered.  If I view the source from the browser, the pushbutton has an ID of "ctl00_ContentPlaceHolder1_details_btnOvpymtNotes".  If I try to find it using something like Page.FindControl("ctl00_ContentPlaceHolder1_details_btnOvpymtNotes"), it returns Nothing.  If I try to find the control with the ID of "ContentPlaceHolder1" using something like Page.FindControl("ctl00_ContentPlaceHolder1"), it also returns Nothing; however, if I try to find it using Page.FindControl("ctl00").FindControl("ContentPlaceHolder1"), it returns a System.Web.UI.WebControls.ContentPlaceHolder object. 

Code for the abbreviated version of the user control is as follows:

<asp:UpdatePanel ID= ...