A dryer can take a long time to dry clothes if the vent that leads outside is blocked. This causes moisture-laden air to become trapped in the dryer with no place to go.
A kink in the dryer ducting could affect the ability of air to flow freely into and out of the dryer, which can make clothing dry much slower. Another culprit could be a clogged lint filter. The filter should be cleaned out after each use. The dryer's lint filter should also be occasionally washed with warm soapy water to remove fabric softener residue from the screen. This helps prevent build-up that could lead to clogs in the dryer duct or vent later on.
A tripped fuse could also be the cause of a dryer that dries too slowly. The majority of household dryers use two fuses or circuit breakers to run properly. It is possible for one of them to trip, which would allow the dryer to keep running but not heat properly.
Another issue is that the dryer could simply be too full. Having too many clothes inside a dryer will cause it to dry more slowly. Extremely wet clothing could be another culprit. If the clothing placed in the dryer is wetter than normal, it will not dry as quickly.