Megamenu cannot move submenus

Hi everybody. I am trying to create a menu similar to the one that I can see in Purity III demo.

I have a menu voice in the main menu with two submenu. For example:

Mainmenu
-Menu1
--voiceA
--voiceB
--voiceC
--voiceD
-Menu2
--voiceE
--voiceF
--voiceG
--voiceH

what I want is to have a submenu where Menu1 and Menu2 are splitted in two different columns.
So, I open megamenu, click over "Mainmenu". "Menu1" and "Menu2" are displayed in a single column. So, I enlarge the pixel size of the tide (850px), then I create another column. But when I click over Menu2, I cannot move it. I cannot click over the arrows.

If I try to do it in the administration panel of Purity 3 demo, I can simply move each submenu clicking over the arrows. What's wrong? Can someone help me?