QCombobox not show up scrollbar

QCombobox not show up scrollbar when setting style sheets:

QComboBox {
combobox-popup: 0;
background: #E6E7E8;
color:#404040;
border: 1px solid #848483;
border-radius: 5px;
}

QComboBox::drop-down {
border-image: url(:/Buttons/images/buttons/DownArrow.png);
width: 40px;
margin:3px;
border-radius:5px;
}

QComboBox QListView {
background: #E6E7E8;
color:#404040;
border: 1px solid #848483;
border-radius:5px;
}

/* Width of the scroll bar */
QComboBox QListView QScrollBar:vertical {
width: 40px;
}

/* Surfae where the handle cam move */
QComboBox QListView QScrollBar::add-page:vertical, QComboBox QListView QScrollBar::sub-page:vertical {
background: #404040;
}

/* Down arrow of the scroll bar */
QComboBox QListView QScrollBar::down-arrow:vertical {
border-image: url(:/Buttons/images/buttons/DownArrow.png);
}

/* Up arrow of the scroll bar */
QComboBox QListView QScrollBar::up-arrow:vertical {
border-image: url(:/Buttons/images/buttons/ArrowUp.png);
}

/* Bottom and up part surrounding the arrows */
QComboBox QListView QScrollBar::add-line:vertical, QComboBox QListView QScrollBar::sub-line:vertical {
background: #E6E7E8;
border: 1px solid #848483;
border-radius:5px;
}