At 6 miles miles from a full power TV transmitter you shouldn't be needing a clear line of sight. You are probably having problems with multipath interference and signal overloading. I suggest getting a directional antenna and variable signal attenuator. If there are any signal amplifiers, get rid of them. At 6 miles away from the transmitters they will be completely unnecessary and will simply overload and distort the signal.
The biggest problem with DTV is you almost need to be an engineer to get it work right. With analog if the setup is wrong you could still at least get a viewable picture use that to determine the problem.