As I understand it, the first couple of hours of OTA DTV channel information in the EPG comes from PSIP data broadcast by the stations. If those stations don't broadcast it or don't broadcast it to standard, No Info in the EPG. Beyond that, the EPG integrates guide information from what it discerns are the analog channel equivalents. If it can't determine an equivalent, No Info in the EPG past a couple of hours. Sometimes I see EPG data for a DTV sub-channel go out a couple of hours, then No Info, because there isn't an equivalent analog channel. Also, you need to be on a DBS channel for a while ever so often so that the EPG can refresh itself. Otherwise, you will see a lot of No Info.
I hope this helps. Personally, I pretty impressed that they can do what they do at this point in DTV evolution.