Not working properly with TB Mega Menu

I recently have installed TB MegaMenu on a site with Nexus Theme, but I have a weird issue with its responsive function.

When the screen width is larger than 979px everything works fine!
On a screen size 768px - 978px, menu appears itself always extended, which is really annoying!
On sizes below 768px, it works fine again, with only the menu button displayed.

Any help would be appreciated.

Thanks