No reflective material is perfectly reflective, and any scratches or soot or whatever will increase the amount of heat transferred from the laser to the missile

No reflective material is perfectly reflective, and any scratches or soot or whatever will increase the amount of heat transferred from the laser to the missile. Heat shields are more effective, but it's unlikely that missiles can carry enough to resist lasers while still remaining effective, similarly to how missiles could carry armor against CIWS, but then they'd be too heavy.