Dirt may be escaping from your vacuum due to one or two things:
'''1.''' The filter on the inside of the dirt trap bag may be loose or unlocked. This may cause dirt and dust to flow freely and escape through either the detachable portion of your vacuum or the end by the head and brush roll.
'''2.''' Your handheld portion which holds the filter may be loose or detached at a certain point. Re-check to make sure that it is secure and fastened.
Hope this helps.
Ben