Flash builder 4.13 list scroller does not work

In the following code, the scrollbar for sourcelist does not appear. I tried to set scroll policy to on. The scrollbar appeared, but it was not active.

<s:HGroup width="100%">
                  
                  <s:RadioButton id="linreflistbu" groupName="listgroup"/>      
                  <s:List id="linereflist"  dataProvider="{upent.linereferencelist}"   click="linereflist_clickHandler(event)" width="40%"  
                              height="40%"
                              dragEnabled="true" itemRenderer="renderers.nobelrenderer2" />
                  
                  <s:RadioButton id="treelevelslistbu" includeIn="mainstate" groupName="listgroup"/>
                  <s:List  id="treelevellist"  width="60%"
                               dataProvider="{upent.nobel}" includeIn="mainstate" itemRenderer="renderers.nobelrenderer2"   height="30%"
                               dragDrop="true" dropEnabled="true"   />
                  
                  
                  <s:RadioButton id="resourcelistbu" includeIn="sourcestate"    groupName="listgroup"/>
                  <s:List id="sourcelist" dataProvider="{upent.sourcelist}"
                              creationComplete="sourcelist_creationCompleteHandler(event)"    
                              changing="sourcelist_changingHandler(event)" itemRenderer="renderers.resourcerenderer"    height="30%"
                              width="40%"  includeIn="sourcestate"/>
                  
            </s:HGroup>