Please see my posts here:
https://www.theanswerbank.co.uk/Technology/Question1596727.html
(GG's post was about different channels but the same information applies in both cases).
In short, your aerial 'resonated' well enough with the old frequencies but not with the new ones. Fitting a signal booster should solve your problem. (You'll need to do another retune after installing it. If you're offered the choice between 'adding' channels or 'replacing' the, choose the 'replace' option. It's important that your TV set carries out a completely fresh scan).