Same happended to me. But, first of all, after I upgraded to 14.04 I didn't have this problem, it happened like two days ago after an update. The point is that I solved it by applying a solution I once knew worked for 13.04. Its the following:

• Run the dconf-editor:

  
  
  

  $ dconf-editor
  
  

  
  
  

  (if you don't have it install it by: sudo apt-get install
  dconf-tools)




• In the pop-up window go to the list on the left and open "core", which you find in the following order:

  
  
  

  org -> compiz -> profiles -> unity -> plugins -> core




• Once selected "core", on the right find a key named "active-plugins", and change the order of the last three entries in
  the "Value" field, so that the last ones are 'unityshell', 'expo' and
  'scale'. For ease of the procedure, delete all the entries in the
  "Value" field, copy the following lines and paste them in the "Value"
  field:

  
  
  

  ['core', 'composite', 'opengl', 'copytex', 'compiztoolbox', 'resize',
  'mousepoll', 'wall', 'widget', 'grid', 'imgpng', 'snap', 'move',
  'place', 'regex', 'vpswitch', 'animation', 'session',
  'unitymtgrabhandles', 'workarounds', 'fade', 'ezoom', 'unityshell',
  'expo', 'scale']




• Reboot.