Freeview channels are transmitted in 'bundles', called 'multiplexes', with each multiplex being on a different frequency. In an ideal world you'd have a separate TV aerial on your roof for each multiplex, with the lengths of the elements exactly matched to the relevant frequencies (to provide perfect 'resonation'). That's clearly impractical, so all TV aerials provide only a 'rough match' to each of the incoming frequencies, meaning that they're often far better at receiving some multiplexes than others.
Further, the strengths of the signals arriving at your TV aerial don't remain constant. They're affected by all sorts of things, including the 11-year sunspot cycle, individual flares on the surface of the sun, the length of the day (affecting the height of the ionosphere above the Earth's surface), atmospheric pressure, precipitation, local interference, etc, etc.
ITV3 and 5USA are both bundled into the 'COM4' multiplex. Your aerial clearly isn't particularly well-matched to the frequency used by COM4, meaning that, although it can 'pull in' channels when signal strengths are good, it can't handle them properly when the signals are weaker (in this case, almost certainly because of the current weather conditions). You might find that you experience similar problems with other COM4 channels too, such as Quest.
The usual solution is simply to fit a signal amplifier:
www.argos.co.uk/product/5713543
(You'll also need a fly lead to go between the amplifier and your telly:
https://www.screwfix.com/p/fly-lead-coaxial-plug-to-plug-1m/41720 )