What is at the top of the image?  Is there ANY blank space at all, for instance, did you leave a 5 or 10 pixel border?  This will cause what you are experiencing.  Essentially, your mouse has moved off the dock, so it's autohiding.  I have this problem myself when I create or use docks that have a blank border before the actual graphic.  Be faster with the mouse is all I can tell you.  When you hit the edge of the screen where the dock is, quickly move it down a little so the mouse will end up on the extended dock.  Or, don't use a border...