If its all the time, then the rotor may not be parallel to the mount. In this case the mount needs to be faced.
orrr
You could have a sticky piston that is causing one pad to drag on the rotor .
orrrr
The caliper needs to be moved away from the rotor and centralised ( this should be relatively easy to spot ) The rotor should be runing back in the middle of the cut-out in the caliper.