Mirror finish is good. If it's not mirrored, then it's not bedded in. Sanding the disc will make the brake worse, and you'll have to bed it in again.
Sounds like contamination, do not use brake cleaner or this will ruin the brake!
Cold, clean water is the best thing. Splash it all over the rotor, the drag the brake on and off gently while riding at slow jogging pace, just 'scrubbing' not pulling hard. Add more water, repeat, keep going until the water on the rotor runs clear. Let it dry then away you go.