In about 15% of cases, it proves impossible to get a full-thickness hole in the iris.  This is more of a problem in dark irises (which have a thicker layer of pigment to get through).  The problem is that once we start doing the laser, the view of the hole we are trying to make gets obscured by debris from the laser pulse.  This means that the hole cannot necessarily be completed in one sitting.  In other cases, the hole is patent for a few months, but closes/heals over time.  Further treatment is then necessary.