Widgets not Staying in Sidebar

Hi, I am building a new theme for my site and I have come across a problem. I am onto building the sidebar and I have registered the sidebar in functions.php and called in in sidebar.php with dynamic_sidebar();

But when I drag widgets to the sidebar in the site admin they don’t stay. i.e. when I refresh the widgets page, the sidebar is shown as empty again.

I have been racking my brain as to what might be causing but I cant work it out. Any help would be much apreciated.
Matt