It's quite possible for QAM channels that were available to become temporally unavailable, for specific customers.
Comcast corporate policy is to provide all the local HD they carry in clear QAM. On a regular basis they also do system maintenance that can move the local HD channels to another QAM channel.
When this occurs the first thing to do is rescan for channels and this usually fixes the issue.
On some occasions, an individual customer may have a preexisting frequency trap on their incoming coax cable. Traps were used by Comcast to limit the channels received, based on what the customer was paying for. Comcast no longer deploys traps, but they may still exist in the field.
If the trap is for a frequency range the new QAM channels are moved to, a customer will not get the channels they were previously getting. In this case they need to contact Comcast and have them come out and remove the trap. This assumes they are a subscriber at a minimum to Limited Basic, the least video package that Comcast offers.
Some Comcast Internet only customers may lose some or all of the local HD channels they were getting when system maintenance is performed, if a trap is in place. Since they are not paying for video service, they can't really call to have a trap removed to solve a video problem. In most areas a Comcast customer can get Limited Basic video & Internet for the same or slightly less price than Internet alone, which may not be well known.
Another issue is that very few Comcast CSR's even know what QAM is. When a customer calls about having issues with QAM, you may well get told 'you can't get any HD without one of our boxes'. It takes patience and probably multiple calls to find someone who knows what QAM is and that can help solve the problem. Try escalating to a supervisor or ask for a technical supervisor.