The outside vent opening of the dryer duct sometimes has a flap that opens and closes. If this flap isn’t opening while the dryer is running then it is most likely clogged. Dryer ducts are also favored by critters (birds, rodents and reptiles) to hide because it can stay nice and warm for them. If this outlet doesn’t have a flap and it’s just an opening sometimes birds make nests inside this and cause clogs.