Your candle may be smoking because the wick is too long and it needs to be trimmed to the recommended 3mm.
If the wick has become too long, small amounts of unburned carbon particles (soot) will escape from the flame as visible wisps of smoke. To avoid this, always remember to keep the wicks trimmed to approx 3mm before you light your candle, as well as during use.
Also, make sure there are no open windows or drafts when burning your candle, as this will disturb the airflow causing the flame to smoke, blackening the glass. In the case of blackened glass; extinguish the flame, allow the wax to cool to room temperature, wipe the blackened glass with a dry paper towel, relight, and your candle is as good as new.
If a candle is continually flickering or smoking, it is not burning properly. Extinguish the flame, allow the candle to cool, trim the wick to 3mm, make sure the area is draft-free, then relight and enjoy.