Widget dragging not working

I changed isDraggable = true in WidgetContainer.mxml and draggable="true" in viewer:WidgetTemplate tag in my Widget mxml files.

Dragging Widgets works great when there are less than 4 open widgets, but when there are more than 3 opened widgets dragging a widget does not work most of the time. All my widgets are inside vertical layout widgetcontainer widget group tag in config.xml.

Does anyone have the same problem as me? If so, would you please share your solution with me? Thanks,