S3 Tivo, Comcast, No HD

I am using Comcast cable in Florida with a Tivo S3. I also have Comcast cable boxes in bedrooms. One channel, SNY, channel 785, shows in HD on TVs with comcast boxes but not on TV with Tivo. Could it be a problem with the cable cards? They were installed in April of 2010.

I haven't called Comcast yet. I am hoping to avoid that because it is always such a dreadful experience.

Thanks for any help.

Margaret