Does that bike have mechanical or hydraulic brakes and which brand?

Not an expert, but if the pads are organic and not bedded in well, there will be a difference in the sticky layer from point to point on the rotor which can lead to chatter. Also any minute contamination may cause chatter. But your LBS should have noticed if that was the issue. Then it could be just the adjustment being off. Some of them can be tricky.

I found this tip somewhere on this forum:

"You need to clean the rotors with brake cleaner. Post cleaning, go about 15mph and slow to about 7mph, do not stop. Do this about 10-15 times. This will spread your pad compound around your rotors and set a base layer of organic goo on your rotors. This will help with noise because you get vibrations (noise) when your pads slide over goo covered parts of you rotor and non goo covered parts. Once you have done this, you need another 10 or so stops from 15+mph to hard stop. This will bed your pads into to your rotor by cementing the goo you applied onto your rotors. Your LBS should have covered this with you when you bought your bike. Cleaning your rotors will help this process."