How can this be fixed?

Misaligned antenna (aerial).

T
July 2015

Probably nothing - maybe...

The set works fine on channels likely to have a good strong signal - some channels - you do not say which channels - are poor - this will be because of a weak signal - the electronics not being able to make proper sense of it.

Either the weak signal will be down to defects in the aerial and cabling to the set or - which is more likely - down to a weak signal from the transmitter.

Not all broadcast transmitters send out all the channels - your bad channels my originate from a station much further away than the station of the good channels.

The set is fine - some signals are weak.

You might do well to discover your nearest TV transmitter and find out what it's sending out - chances are it is not bothering itself with your weak channels - they will probably come from far away.

Good luck...
July 2015