1: The antenna may not pick them up. Plug it into your TV and scan for channels to see if the antenna is picking them up.
2: Any time there is RED on the weather radar between your location and the satellite (even if its miles away, but high enough), the signal will get blocked out, and you will lose the satellite signal until the weather cloud passes. This is usually quite a short time. The SD channels have better coverage, so sometimes switching to the SD channel will allow you to continue viewing.