I had a similar issue with HD channels not displaying last year when using my mini digital box. It turned out to be signal loss in the house that was causing it. Cox install a powered signal booster and that helped. Signal loss can be caused by the coax splitters in the house and poor connectors. Cox did eventually come to the house again and analyze the signal as it comes into the house and found that my coax outside the house had been damaged by the contractor that installed new siding on the house. Cox replaced that cabling and my signal loss issue was solved. Didn’t even need the powered signal booster anymore.