you need to tighten them. It's a pretty universal process. It can be fairly fiddly, having another person help you tighten it whilst you hold the pads just above the wire. another guess at this is that you may need new brake pads, they are consumable and can wear down. tightening them works until you need to buy new ones, it's basically re-calibrating them. The closer to the wheel they are, the more likely they are to stop it, so I recommend you look up a guide on google "How to tighten bicycle brakes" or how to calibrate them. I'm not a mechanic, but I have experienced this issue with my bicycle!