Hi Eddie,
The problem here is that you have many items on the main menu and the space in portrait view is not enough show them and it breaks to the next line (behind the slideshow now), you can consider to reduce the font size of these menu items in this portrait view by this custom CSS:
@media (min-width: 768px) and (max-width: 991px) {
.navbar-default .navbar-nav > li > a {
font-size: 12px;
}
}
The current font size is 14px, you can change to the value which suits your need.