three level menu is hiding

Hi,
I’m using twenty Eleven theme. I have a three level menu, and when I click on the third menu, both the second and third level menu they hide:

Home
Portfolio
– Customers
– – Company
– – Workers

Every time I click on “customers” or “workers” all the menu shrinks. This is the part of the CSS that is involved:

What Should I add, please heeeelp!!

#menu {
        margin-bottom: 10px;
        margin-left: 13px;
        margin-top: 3px;
        padding-bottom:10px;
        width: 175px;
    }
    #menu ul li a{
        color: #999999;
        font-family: Arial;
        font-size: 13px;
        text-decoration: none;
        font-weight: 700;
    }
    #menu ul li.current_page_item a{
        color: #00A2E4;
    }
    #menu ul li{
        border-bottom: 2px solid #DEDEDE;
        padding: 2px 0;
        line-height: 16px;
    }
    #menu ul li.current_page_parent a{
        color: #00A2E4;
    }
    #menu ul li a:hover {
        color:#00A2E4;
    }

    /* BOF submenus */
    #menu ul li ul {
        padding-left: 15px;
        border-top: 2px solid #DEDEDE;
        display:none;
    }
    #menu ul li ul li{
        border: none;
        line-height: 18px;
    }
    #menu ul li ul li a, #menu ul li.current_page_parent ul li a{
        font-size: 11px;
        color: #999999;
    }
     #menu ul li.current_page_item ul li a{
        color: #999999;
     }
    #menu ul li.current_page_parent ul, #menu ul li.current_page_item ul {
        display: block;
    }
    #menu ul li ul li.current_page_item a {
        color:#00A2E4;
    }
    #menu ul li ul li a:hover {
        color:#00A2E4;
    }

Thanks in advance.