Is the dyer getting plenty warm inside? If yes next Make sure dryer hose is not kinked .keep lint cleaned off and out of dryer, especially the back. Lint is a fire hazard.
Does your outside dryer exhaust vent have a hood on it to prevent rain from getting inside it? If not it may be blocked partially or completely with water. It's also suppose to have flaps to allow air out and yet let nothing in . Lint can accumulate on it. Keep it cleaned off so they can work properly. If the vent hose leading up to the vent outside has and curves or turns in it lint or water ( if rain gets into it) will accumulate and make clothes take longer to dry or not at all. I'd recommend a wet dry shop vac with adequately long enough hose to run down the dryer exhaust line and vaccum out.
Some people run the dryer exhaust hose thru the floor or wall and exhaust under the house or on an attic. I would not recommend this. Dryer exhaust contains lint ( fire hazard) and moisture ( breeding ground for mold , mildew, and rot.) It needs to be vented outside.