This seems to be caused by some annoying interaction between the OS and the mouse's dongle. A search for this recommends using a USB extension cable with the dongle on the end. Since I moved the dongle for mine to the end of a USB 2 extension, and then positioned it further away from the hub, it's been more reliable, but still not 100% reliable. At other times, you need to disconnect, reconnect the dongle.
Symptoms of when it doesn't work:
-Dock doesn't appear when you move mouse to bottom of screen.
-Any mouseover behaviour on menus or links doesn't work.
-The mouse pointer changes to a pointy finger in the current app window when there are clickable links on the browser window *behind* the app.