Menu drops one level, then goes horizontal

I have a simple menu that does drop down on hover, but only one level. The sub’s then spread horizontally.

So: menu item | menu item | menu item
|
sub item – sub-item – sub-item

Whereas I want: menu item | menu item | menu item
|
sub
sub
sub

This is the relevant code:

/* MENU */
 .topnavigationgroup {
    margin: 0px;
	padding-right: 40px;
}

.topnavigationgroup ul {
    width: 100%;
    float: left;
    margin: 0;
    padding: 0;
    list-style: none;
}

.topnavigationgroup ul li:last-child a{border-right:1px solid #F5DC32;}

.topnavigationgroup li {
    float: left;
    color: #FFF;
    font-size: 12px;
    font-family: Arial, Helvetica, Verdana, sans-serif;
}

.topnavigationgroup li a {
    display: block;
    padding: 2px 10px;
    text-decoration: none;
    color: #FFF;
    border-left: 1px solid #F5DC32;
}

.topnavigationgroup li a:hover {
    color: #FFF;
    background-color: #259999;
}

.page_item {
    text-decoration:none;
    display: block;
    color: #FFFFFF;
    background-color: #3f6997;
}

.current_page_item {
	background-color: #FF6E3D;
}

Any help is greatly appreciated.