Hello,  On several occasions now, I have found the windshield has fogged up or frozen, and have been unable to see outside.  I have worked through the icing procedures, but have had no success - only option is to fly at low altitude with above freezing temps.    Tonight I was attempting to depart from KDLH and they had freezing temps with snow. I deiced as per normal procedure and ensured the icing system was working and turned on but the windshield would not defog or deice.