Flex 4 Scroller sometimes doesn't show

Ok so I have a simple unskinned Scroller component , the problem I'm having is that at aparently random times the vertical scrollbar dissapears. I can still scroll the content with my mouse wheel but you can understand how that's just not enough for a quality product.

here's my code

<s:Scroller id="scroller" width="100%" height="150" horizontalScrollPolicy="off" verticalScrollPolicy="on">
    <s:DataGroup
        dataProvider            = "{_myDataSource}"
        width                   = "100%"

        clipAndEnableScrolling  = "true"
        itemRenderer            = "MyItemRenderer"> 
        <s:layout>
            <s:VerticalLayout
                useVirtualLayout    = "true"
                rowHeight           = "30"
                gap                 = "10" 
                horizontalAlign     = "left" 
                paddingRight        = "10"
                verticalAlign       = "top开发者_开发问答"/>
        </s:layout>
    </s:DataGroup>
</s:Scroller>

Is it a flex bug? As I've said it seems to happen randomly. The only thing I do know is that it only triggers when and if the data source array gets changed. For instance in a chat window where messages keep comming in. This occurred for me in multiple places so I'm thinking it's not the ItemRenderer that's causing the problem.

Setting the height of the scroller to 100%, and placing the scroller inside a component of height 150 ,Still, it seems "100%" should work