Sub menu problems in Twenty Ten

I have recently installed WordPress and at first had no problems creating the navigation menu. I followed the instructions for creating submenus -‘you can also drag a menu item a little to the right to make it a submenu, to create menus with hierarchy. Drop the item into its new nested placement when the dotted rectangle target shifts over, also a little to the right. Don’t forget to click Save when you’re finished’ – and this worked.
I only encountered a problem when I added additional menu items at a later date. I was able to drag the submenu items to the right but as soon as I saved the menu these items moved back to the left. Unfortunately I made things worse be deleting the menu and starting again and now all the submenu items keep moving back to the left.

For the time being I have created a top level menu with no submenu items and I am relying on html links from each parent page. Can anyone help with this problem. Thanks.