The ratio of air inlet cross section and air outlet cross section is often a weak point of the smokers (a deficiency, which however hardly occurs with brand products.) and also the mouth opening is partly too small – either for optical reasons or because of smoking residues. These should be removed from time to time. Pipe cleaners can be used for this.
Or warm the inside of the smoker over a wax candle, then try to loosen the residue.
Also, the body should not be too cold. This cools the smoke inside too much and suffocates the embers.
Often one can achieve the desired success by small changes at the smoker.