A heat pump dryer moves the same air around a closed loop and pulls the moisture out through a condenser, so it lives or dies on airflow through that condenser. The lint filter catches the coarse stuff; the fine lint goes through it and packs into the condenser fins, and once those are loaded the machine cannot shed heat or moisture. Cycles stretch out, then stop early with clothes still wet.
We clean the condenser and the evaporator, check the drain pump and the condensate reservoir for blockages, and test the compressor and its thermistors if the loop is not reaching temperature. Compressor faults on these are a bigger job than a vented dryer, and we quote them before touching anything. See dryer repair.