If you have channels missing on freesat it can be any number of things.
Most common I find is the plastic white cover on the lnb has perished and it lets water in. Tap the end gently and your finger will go through the plastic. New LNB needed
The dish or bracket has rotted and has dropped slightly losing signal. New dish
The F connectors on the lnb need replacing as they have got weathered. New F connectors needed
The dish wasn’t tightened up enough when fitted and is very slightly out of alignment.
HTH