Responsive Menu disappears

Hi, I discovered that the menu disappears when I make the browser window smaller than about 933px till the with of 744px - here the smaller responsive menu appears correctly. Since when this happens, I dont know. Normally it worked fine. I use the great Catch Box Theme (as a child theme with some changes at the css) on http://www.rc-tri-run-weiz.at Could you help me please. Thanks.