My AM21 gradually lost sensitivity over time (with both the signal strength numbers and the ability of the AM21 to receive stations gradually decreasing), to the point where I had to replace it. At first, I lost my distant channels that were 50-65 miles away. Then, I lost another station 25 miles away (it first started to pixelate, right at the threshold of being received, then I lost it altogether). This was a station that initially had an 85 signal strength that gradually declined to 34, then disappeared altogether on the meter. Then, I started to lose one or two of my local channels (8-12 miles away). Signal strength numbers slowly declined through this time on the AM21, but my antenna was fine, as my tv's tuner still received everything just fine, with no signal strength change on the tv's meter.
I called DirecTv, and was told that there is no warranty (at all) on the AM21, that they would not replace it, and that I had to purchase another one from them for $50 plus $3 tax. Since I could no longer reliably record shows over-the-air with my AM21, I caved and bought another unit. As soon as I hooked up the new AM21 unit (using all of the original cables from the previous AM21), it instantly worked perfectly (as good as the previous unit when it was brand new, getting all of my 50-65-mile stations again, and pretty much equivalent again to my tv's tuner). I had noticed that the AM21 was rather warm from sitting atop my HR21 receiver at the time, so I moved the location of the second AM21 unit, just in case heat might have been the cause. I did eventually get that $53 credited to my account later during a subsequent phone call with DirecTV's customer service about another subject, when they asked me whether I was satisfied with their service overall and I brought up my previous disappointment with the AM21 situation.
Unfortunately, my second AM21 (now almost two years old) is now showing the first stage of this same gradual decline, as a recent recording showed a blank screen due to insufficient signal strength on my 25-mile channel (which comes in at 83 on my tv's signal meter, and used to be at the same number on my AM21). So it looks like I'll be calling DirecTV again soon. A few other people have reported this issue as well, so maybe your AM21 is experiencing the same problem.