Well, the simplest explanation that I can think of is directional. Do you have the antenna pointing in the same direction every time you hook it up? If your main room is downstairs or in a room that is opposite the side of the house where the signal is coming in, then the reception could be effected. Do you know where the broadcast tower is in relation to your house? You might try pointing the antenna directly at the tower (and out a window if you have thick walls in your house).