'Menu' Not Working Properly on Mobile View

Hi Need some help here. I realized the 'Menu' function is not working properly on mobile on certain pages. For eg: http://www.investphnompenh.com/the-skyline-cambodia/ Although the menu bar is working fine on desktop view, but when it comes to mobile view, it doesn't appear correctly when clicked on the 'expand menu' function. You will understand more when u visit that page on mobile. Please kindly advise. Best regards.