I know VERY little about this, as we just got our first HD telie, and shhh, don't tell, we are currently running it on a tabletop rabbit ears antenna.
Al explained to me, that the HD is more sensative, and you have to be more "ON" with your antenna placement.
That said we are picking up stations from 3 major markets, and there is at least one all the time weather station. For someone with no cable, no dish, etc. this makes ole Carol grin with glee!
But, I think the answer to your situation is minute adjustment of the antenna??
Now, the techies can all come on and tell us the real reason!