OTA digital antenna reception - problem

I just installed a new HDTV OTA directional antenna (non-amplified) on my roof. I aimed it at 48 degrees, according to this chart for Santa Monica, CA. Theoretically, all the stations should come in cleanly since they're all broadcast from the same place, Mt. Wilson. So much for theory! I need to aim the antenna in different directions to pick up different stations. When they come in, the quality is excellent. But, I can't get them to all come in together. What's going on? The stations still come in w/o being amplified so that shouldn't be the problem - should it? Even if I was to get an amplified antenna, getting electricity up to the roof wouldn't be easy. Suggestions?