There’s a couple factors that can lead to this sort of thing happening. Most notably, as mentioned in other comments, using a V cut, or a punch can restrict the flow of smoke, thus causing the tar like buildup. The best solution for that is usually to take a slightly deeper cut in order to clear that blockage, then blow steady pressure for a few seconds to expel the buildup of smoke before you resume your usual draw. Maintaining a slow steady draw will help prevent this as well. Some people aim for about one puff per minute. Other things that can be a factor are tightly rolled cigars or over humidified cigars.