This is because its inner width is 0, since only fields add to the outer width:
.divider-vertical { height: 40px; margin: 0 9px; border-left: 1px solid #F2F2F2; border-right: 1px solid #FFF; }
You need to add an inner width to it, for example.
.navbar .nav .divider-vertical{ width: 20px; background-image: url("img/nav-div.jpg"); }
You will probably need to reduce the margins of the element to compensate for the added width (if you need .divider-vertical to remain 20px width).
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