Scrollbar not showing, but panel does

I have a table surrounded by an with the a 开发者_JS百科css class:

DIV.scrollableTable {
 overflow-y: scroll;
 height: 100%; 
 width: 100%;  
}

when I load the page, the scroll panel is present, but the bar isn't. And in order for me to scroll I have to select everything in the table and move the mouse down. Any idea what the problem could be?

In order for the scroll bar to show, the content of DIV.scrollTable must be higher than "DIV.scrollTable".