If they are squealing and not stopping very well, that is an indicator that the rotors have been contaminated with some type of oil. Try cleaning the rotors with some acetone or brake cleaner and abrade the rotors with some 220 grit emery cloth.
There are numerous ways that the rotors can be contaminated. It could be from lubing the chain, oils from the road mixing with water and splashing on the rotors(if you ride in the rain), overspray from sunscreen, etc.