I'm assuming you have new discs and pads, it may be the coating on the discs has not fully burned off. Do some more taxiing and do a lot of light brake applications and turns to "burn" the brakes in. You don't want to overheat the brakes. A light application followed by some cool off time. It sometimes takes a bit to set them properly. Too much heat will warp the discs and even cause a fire, in extreme conditions.
Hope this helps,
Roberta