Pre-emergent applications will never be 100% effective. The pre-emergent application creates a barrier that prevents weed seeds from growing. Anything that brings soil and weed seed into contact or the barrier is disturbed will allow the seed to germinate. Broadleaf weeds can only be controlled when they are actively growing.  Weed seeds germinate at different times of the year which is why more than one treatment is needed. There are also times when hard-to-control weeds will require repeat applications.