For Carolegif:
If you've not already got a signal booster, go out and buy one. If you have, get an aerial fitter to upgrade your aerial. (Digital TV signals are bundled into 'multiplexes', each of which is on a different frequency. You ideally need a different aerial to 'resonate' with each frequency but that's not practical, so any aerial is likely to be a better match to some multiplexes than to others. Yours is fairly well-matched to the BBC multiplex but not to the one which carries the ITV channels).