Hi
It looks like there is not enough space to show all items in one line when screen size is small.
You can use this custom CSS:
@media (min-width: 992px) and (max-width: 1400px){
.t3-mainnav {
padding-right: 0px;
}
.navbar-default .navbar-nav > li > a, .navbar-default .navbar-nav > li > span {
margin: 0 10px;
}
}