Reception problems with an HBD8X.

I'm trying to get my 80 year old mom away from Directv. She only lives 5 miles from all of the tv towers, and all towers are in the same spot so I was thinking it would have been as easy as rabbit ears for her to be able to pick up all the stations. Of course that didn't happen. First I tried an RCA ANT751 which would not pick up channels 5,11,or 21. No matter how I aimed the antenna, I couldn't get over a 32 signal on the Tivo. I looked on the antennaweb site and was amazed when I saw her house was in a violet zone! Now the tv towers are blocked by part of the mountain that they are on, meaning that from her house I can only see the top 20% of one of the many towers, but I figured at a distance of less than 5 miles this shouldn't matter THAT much. I figured I'd go all out and get an HBD8X because of it's ability to pick up weak signals. Imagine my shock when after hooking up this beast channel 5 and 21 are only at a 35% signal. The good news is most of the other channels are in the 70's range now, where as before they were in the 50's.

The only thing I can figure is due to the long cable run (50') from the antenna to the tv, I'm losing too much of the signal? Do I really have to get a pre-amplifier with an HBD8X to pull in channels from towers that are less than 5 miles away? I realize the mountain blocking the tv towers to her house is a problem, but I can't figure out why some channels are ok and others are not, even though all of the towers for all channels are right next to each other. All of this is complicated by the fact that she lives on the other side of town and my time to get over there to work on this is limited.