Technically all fireplaces will soot in time. This is because the vents open to the outside and wind and dirt will get into the fireplace. When you turn on the fireplace the moisture is burned out and the dirt will cling to the glass. For this reason it is suggested that you remove your glass and clean it at least once a year. A second possibility is that something is blocking the flame. The logs could be impinging the flame improperly or there is a possibility that a spider nest or other obstruction in the orifice or burner tube is restricting gas and air flow.