I have 2 Tivo HD XL boxes with CableCards and I receive all channels one one box but am missing channels on the other box.

The Tivo Series 3 uses two CableCards, one for each tuner. It is possible that the CableCard that is responsible for receiving channels on Tuner 1 is defective, has become corrupted, or otherwise is not paired correctly. Call your cable provider and have a tech look at it.

I've never, ever heard of this problem - having been a happy TiVo user for over 15 years. The very first thing I would do is call Comcast and tell them what is occurring. How long had everything worked OK before this happened? If things were OK for a long time before this, and if your TV carrier was, likewise, Comcast for a long time; and if you haven't gotten a new TV or cable box for a long time, then the problem must rest with Comcast. It must!
In our home, with the number of movies, shows/sporting events/etc. which we record, that would be totally unacceptable!

Try this: Unplug everything (with everything turned off). Make sure your TiVo remote is good (fully charged batteries and the code is right for your TV/cable box), Wait for 10 minutes and plug everything back in, starting with TiVo, then the TV, then any other accessories.

Plug back in and do a complete restart from scratch - like you did when you first set up TiVo.

Make SURE that your receiving channel (3 or 4) is the same on both. It could be that someone accidentally has changed it - are there kids in the house?

If none of this works, get Comcast in there. That's all I can tell you. . .

JB

The end of the IR Cable that attaches to your cable box may have to be repositioned.

Try moving it a small distance (you are already close or you wouln't be receiving ANY channel changing info. from it) and see if that helps.

Good luck!

They're lying to you, to charge you more.
A cable card is a cable card is a cable card.  If your provider stands by their "special card" BS, switch to a new cable provider.
They're probably trying to scare you into getting their DVR instead of Tivo.  And it's not the same, ever. Tivo is far superior to any other DVR on the market, free or not.

As far as I know, you need the cable cards to get HD through the Tivo, because they're what does the HD decoding and output.

Your TiVo is only recording basic cable (i.e., channels it can receive without a cable box). You will need a digital cable box from your cable company to record channels above 99. If your TiVo is a Series 2 you will not be able to actually record in HD either. You should be able to record the HD channels from the cable box, but they will not be HD quality. (This is my current setup and while the picture quality is much better than basic cable, it pales compared to bypassing the TiVo and watching HD directly off the cable box to the TV.)

Yes, the Tivo will just record in standard definition, instead of high def.

You can still hook the two up together, though.

What I would suggest (and have done in the past) is this:
Split the cable coming out of the cable box.
Hook one up to your HD cable box, and the other up to the Tivo.
Hook the output of your HD cable box to one HD input on your TV
Hook the output of your Tivo up to a different (non-HD) input on your TV.

This will allow you to watch HD while recording standard definition, and vise versa.

Running the HD through the Tivo will work, but will limit you to not be able to watch HD through it.

Set up your infrared blasters connected from your IR port on the Tivo and by placing the two lead lights in front of your cable box so it (the cable box) can read the signal.  Then go to Channel settings accessed through Tivo Central and follow the questions. It will ask you to change channels until the remote is working correctly.  This all worked on a different "input" on my Mitsubishi TV (it has multiple inputs.  I have the cable input on a different "input" to make it work.  Hope this helps.  Call TIVO tech assistance because they are the ones who talked me through this.

The solution I use is S-Video from cable box to TIVO, and from TIVO to TV. That gets you the highest quality signal that the TIVO can handle. To watch true HD signal, switch TV from S-Video input to Hi-Def input and tune to HD channel. I have an HD cable box connected to both outputs, your situation may be different.

Feel free to ask for more help.

Jeepster77