In the CSSMenuWriter/cssmw/menu.css file, change the margin for the level UL:
ul#cssmw ul.level-1 {
left: 0px;
margin: 1px 1px 1px 0px;
padding: 1px 10px 1px 1px;
top: 100%;
}
to have a 0px top and bottom value:
ul#cssmw ul.level-1 {
left: 0px;
margin: 0px 1px 0px 0px;
padding: 1px 10px 1px 1px;
top: 100%;
}
the gap caused by the margin is causing the menu to close.