Sounds like your unit is in single feed mode. Does the channel up/down button offer a few channels?
The unit will choose the mode only at initial setup or reset to default.
If just a few channels are offered with channel up/down then perform a reset and watch carefully for an on screen message that two feeds have been detected. If only one then you have a cable fault, probably a simple short due to a badly fitted F connector.
A reset to default retains recordings but does loose your recording schedule and changed settings – so make a note first. A photo is a good and quick way of capturing your schedule.