There are a couple of things you can check. First one is to make sure when you installed the little aluminum phasing wires into the cartridge housing that they are in the proper slots in the middle of the housing. Secondly make sure that the little aluminum phasing wires are not touching any part of the metal boom or metal brackets since that will short out the UHF signal. You can go to www.winegard.com and under the HD8200U there is a place to check out pictures of the proper installation of the phasing lines.
The other problem may be that the antenna needs to be raised higher to receive a stronger signal on UHF. What we finding is that the UHF digital signals require the antenna to be mounted higher than the old analog signals. A good average to use is what ever distance you want to receive in mileage the antenna needs to be mounted in height in feet. Example if your stations are 50 miles away you will want your antenna to be 40-50 feet high.