Change this block of code from your css_additional.css on or about line 30
Code:
@media only screen and (max-width: 783px) to @media only screen and (max-width: 767px)
and see if that rectifies the situation. These are the kind of issues I was referring to when I offered up the larger avatar code. There are so many devices with different screen resolutions that keeping the responsive design working for all of them will require quite a bit of tweaking.