menu items dissapear in main menu in position header top

Hello, The URL is Hagmeijer.nl I want to have the main menu in the header top position. When I position the main menu in the header main position, the menu occurs on the right place. But if I want to click on one of the menu items, the menu bar with items dissapear. On the same site I installed the catch store pro theme and there is no problem. But it is quiet a job to change themes. Is there a solution for this problem? Kind regards Hans Hagmeijer