if its just the one brake not performing correctly it could be something so silly as air in the system which simply requires bleeding, or a duff calliper needing either rebuilding or replacing this could have failed in a couple of ways, either its partially seized which is preventing the piston from squeezing the pads onto the disc or its seized with the pads applied which is overheating the disc and pads resulting in brake fade as the heat builds up meaning the brake performance is ruined. In any case it does require some pretty urgent attention. Brakes are the one thing it's best not to take shortcuts on. While on my apprenticeship the old boy teaching me once said 'Do you reckon 99% in an exam is good?' (We all would say yes) 'Well how about if you 99% of your brakes jobs well and got the last 1% wrong? That definitely wouldn't be good.' Moral of the story is; if you are in any way unsure it is always best to get professional help.
