If it's only those 2 channels that are affected as they are not on the same mux I would try a reboot of the box or a factory reset if the reboot doesn't work.
Reboot the box by turning the power off and on with either the switch on the back of the box or at the plug.
If that doesn't work you could then try the factory reset.
It's only a partial system reset and not a complete factory reset. You are given 2 options,
Choose Keep Recordings, that means you won't lose your recordings and any planned recordings or series links you've set up, you will have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again, also Match frame rate and Standby power consumption will go back to their default settings if you have changed them.