MOT Fail service brake efficiency below requirement

Hi, apparently my 'service brake' efficiency is below the requirement so I failed the MOT.

Obviously the guy at the garage was very keen for me to pay him to strip down my brake system and 'fix' them though they seem to work fine to me.

Any tips on what could cause this?

I did replace the brake fluid not so long ago but maybe flushing more fluid through could sort it?

Thanks