If anyone at Comcast tells you to exchange your DVR box to fix this issue, DO NOT BELIEVE THEM. I have had this problem for many years. It has been happening regularly to me on 8 (yes EIGHT) DVR's.
There is no rhyme or reason to this problem, it is just the luck of the draw as to whether or not your program will record or not. The "recordings" always show up in your "My Recordings" list as fully recorded, but when you go to watch the program it shows up as nothing but a black screen.
Comcast is the worst company I have ever had to deal with. I have been a customer in the same home for 17 years so I know what I am talking about. If I had a choice I would not be a customer. Their representatives lie all the time regarding issues you might be having. I am a very hight level engineer and I know a thing or two about troubleshooting. When I have to go through multiple levels of bullshít from the tech staff just to get to a half-wit that might offer something constructive it really frustrates me.
With all the frustration I have had, the best one happened last week. my wife's DVR in the living room suddenly stopped working. There was no picture on the TV. I checked and it was definitely a problem with the DVR not putting out an HD signal from the Component outputs beyond the "Please Wait!" message at bootup. The standard video out and S-Video signals worked, but no HD picture. I contacted Comcast and they said I had to come in and replace the box. So for the umpteenth time I drove to Rockville to exchange the box. My wife was terribly upset that she lost all her recordings for the fourth time. When I got home and put the new box in and it had the same problem. I called Comcast and they tried to tell me it was a problem with the TV or the cable. I told them I had proven this was not the case and that the TV and cable worked fine if attached to my DVD player. They said they had to send someone. Of course, this took two days. When the tech gets there he determines the DVR is defective and replaces it. It works for the entire time he is there. Problem solved??? Nope! The minute he leaves it does the same thing. Fortunately I caught him before he drove away. After much fussing and again being told it was my equipment I had to insist FIRMLY that he was an idíot as I just proved to him it was not the case. He gets on the phone to his boss or something. 20 minutes later he says, someone had turned of the HDTV signal for that box, as in Comcast reprogramed the box to NOT allow it to output HD signals. Wrong! since I pay for HD (which is a scam in and of itself). The other Comcast boxes in the house still had HDTV signals and worked fine, just this one was turned off, as well as the two other boxes they replaced the original with. So my wife did not need to lose her DVR or her recordings. This was all some screwup at Comcast. Don't even try to guess how for some unknown reason one of my DVR's had the HD signal disabled. Don't even try to understand how no one at Comcast thought to check this first when I clearly told them there was no signal coming out of the HD ports. I am telling you these people are the worse trained I have ever seen.
Verizon just ran FIOS cable through our neighborhood 3 weeks ago. I am on the list for a phone call so I can switch and be rid of Comcast for good. I am not a Verizon fan either, I had their crappy service some 20 years ago and swore I would never do business with them again. Alas, it is them or Comcast and my Comcast bill keeps creeping up and up. I know the FIOS will be cheaper and so I will switch. I have no movie channels and my Comcast bill crept up to $285 until I finally called to cancel and they dropped it back down to $120. Then it started creeping up till it hit $180 and I called again. It is currently still too high at $140.
It's no surprise that in a magazine I saw somewhere last month there was an article on "The top 5 worst customer service companies in America". Comcast and Verizon were both on the list.