I'm assuming that those channels are missing when you use Freeview, Jennyjoan, rather than when you're using the signal from your Sky box.
Freeview channels are transmitted in 'multiplexes', which are bundles of channels sharing a common frequency. Your TV is picking up signals from the 'PSB1' multiplex (which includes all the main BBC channels) and from the 'SV2' multiplex (which has local programming for NI on it). However it's not picking up signals from the 'PSB2' multiplex (which has ITV, Channel 4, Channel 5, etc included).
Assuming that there's not a transmitter fault, the reason will be that your TV aerial doesn't 'resonate' well with the frequency used by the PSB2 multiplex, resulting in signals disappearing when reception conditions are poor (or simply not being present at all). The usual solution is to fit a signal booster, such as one of these:
https://www.argos.co.uk/product/9383027
(You'd also need a fly lead, such as this one:
https://www.screwfix.com/p/philex-coaxial-cable-1m/41720 ). However, if you've got a Sky box anyway, you should be able to receive the missing channels through that.