Only last saved Content page is loaded into ContentPlaceholder elements

Question

I have a master page that defines content placeholders for a header, content and footer.

The master page is formatted from with head and body. It also has a that wraps around the entire main content so that content pages do not have to add the form tag. The part that includes the content pages are like this:

    <section id="header">
        <asp:ContentPlaceHolder ID="placeholderHeader" runat="server">
            header default
        </asp:ContentPlaceHolder>
    </section> 

The content pages are formatted as shown for testing:

<%@ Page Language="C#" AutoEventWireup="true" MasterPageFile="~/Pages/Master/MasterIndex.Master" CodeFile="~/Controllers/IndexController.cs" Inherits="MetisClient.Controllers.IndexController" %>

<asp:Content ID="ContentHeader" ClientIDMode="static" ContentPlaceHolderID="placeholderHeader" runat="server">  
  
HEADER CONTENT
  <asp:Button ID="btnTest" class="btn-submit" runat="server" Text="Button Test" OnClick="btnTest_Click" />
    
</asp:Content> 

The problem is that the master page only seems to load one of the content pages - whichever was saved last. If I go in and ctrl+s the footer, and debug, only the footer shows, same for the others. The content pages CodeFile="" and Inherits="" to the same page controller file, instead of having one each.

Is it supposed to behave like this? What am I doing wrong?