TL;DR
A dryer that needs two cycles almost always has an airflow problem, not a heat problem. Work the restriction from the machine outward: lint screen film, then the housing under it, then the exhaust duct, then the outside vent flap. Overloading and a wet-arriving load from a weak washer spin are the other two common causes.
A dryer does not dry with heat alone
Heat is only half of the process. A dryer heats air, moves it through the tumbling load to pick up moisture, and pushes that humid air out through the exhaust duct. If the air cannot get out, the moisture stays in the drum and the clothes never finish.
That is why almost every "my dryer takes forever" complaint traces back to airflow. The heating element is usually working fine. The air is the problem.
Why does my dryer take two cycles to dry?
Because something is restricting airflow. In order of likelihood: a film on the lint screen, packed lint in the screen housing, a clogged or crushed exhaust duct, a blocked outside vent flap, an overloaded drum, or clothes arriving too wet from a washer that is not spinning properly.
Work the restriction in order
Do not jump to the expensive answer. Check these in sequence, and stop when the drying time comes back.
1. The invisible film on the lint screen
This is the most missed cause. Dryer sheets melt a softening coating onto fabric in the drum, and some of it lands on the screen instead. Over months it forms a clear film that blocks air through mesh that looks perfectly clean.
Test it: pull the screen and pour water on it over a sink. Clean mesh passes water instantly. If water beads or pools, there is film. Wash the lint screen film off with hot water, dish soap and a soft brush, then dry it fully before reinserting.
If you use dryer sheets on every load, this will come back. Dryer sheets versus wool dryer balls covers the alternative, and do dryer sheets leave residue explains what else the coating affects.
2. Packed lint in the screen housing
Lint escapes around the edges of the screen and packs the slot underneath it. Unplug the machine, pull the screen, and vacuum the slot with a crevice tool or a long narrow lint brush. Once or twice a year.
3. The exhaust duct
If the screen is clean and the machine still runs long, the duct is the next suspect and by far the most important one for safety. Lint accumulates on bends, in sags, and at the outside vent hood. Failure to clean is the leading contributing factor in home clothes dryer fires. Source: National Fire Protection Association
Clean the dryer vent duct once a year. While you are back there, check that the duct behind the machine is not crushed against the wall, and replace thin foil or vinyl accordion hose with rigid or semi-rigid metal.
4. The outside vent flap
Go outside while the dryer is running on a heated cycle. The flapper should be pushed open by strong warm air. If it barely moves, or it is packed with lint, or someone has installed a wire screen over it, you have found the blockage. Remove screens -- they trap lint rather than pests.
The two causes that are not airflow
Overloading the drum
Clothes dry when air can circulate around them and the tumbling can expose new surfaces. A drum packed past about three-quarters full does neither. The load rolls as a single mass, the outer layer dries and the core stays wet, and the moisture sensor reads the dry outer layer and shuts off early.
Two normal loads dry faster and use less energy than one oversized load run twice.
Clothes arriving too wet
The washer spin does most of the water removal. If the spin cycle is weak, unbalanced, or short, the dryer starts from much wetter clothes and takes proportionally longer. A load that comes out of the washer dripping rather than damp is a washer problem.
Overloading the washer causes this too, because an unbalanced drum reduces its spin speed to protect itself. How full a washer load should be covers the right level.
The moisture sensor is worth a wipe
Most modern dryers use two metal strips inside the drum, usually near the lint screen opening, to sense moisture by measuring conductivity as clothes tumble across them. Fabric softener and dryer sheet residue coat those strips and insulate them.
An insulated sensor reads clothes as dry earlier than they are, so an automatic cycle shuts off with a wet load. Wipe the strips with a cloth dampened with rubbing alcohol a few times a year. If automatic cycles have been unreliable but timed cycles work fine, this is very likely the cause.
When it is a repair, not a cleaning
If airflow at the outside vent is strong, the screen is clean, the load size is reasonable, and the clothes are barely warm at the end of a cycle, the heating element, thermal fuse or thermostat may have failed. On a gas dryer it may be the igniter or gas valve coils.
A dryer that tumbles with no heat at all is nearly always a component failure rather than a cleaning issue, and a thermal fuse that has blown usually blew because of a restricted duct in the first place.
FAQ
Why does my dryer run but not dry the clothes?
Almost always restricted airflow. The heat is produced but the humid air cannot escape, so moisture recirculates. Check the lint screen for film, the housing under it for packed lint, the exhaust duct for blockage, and the outside vent flap for free movement.
Can too many clothes in the dryer make it take longer?
Yes, substantially. A packed drum cannot tumble, so air never reaches the middle of the load and the moisture sensor reads only the dry outer layer. Filling the drum no more than about three-quarters and drying two smaller loads is faster than one oversized load.
Why are my clothes still damp when the dryer says the cycle is done?
Usually a moisture sensor coated with fabric softener residue, which makes it read dry too early. Wipe the metal sensor strips inside the drum with rubbing alcohol on a cloth. If timed cycles work but automatic cycles do not, that is the classic symptom.
Does a longer dryer vent duct really matter?
Yes. Every additional foot and every elbow adds resistance, and manufacturers publish maximum equivalent lengths for a reason. Long runs with several turns also collect lint faster, so they need cleaning more often than a short straight run does.
Should I use a higher heat setting to speed things up?
No. If airflow is restricted, more heat raises the internal temperature without removing more moisture, which stresses the machine and increases fire risk. Fix the airflow instead. High heat also shrinks cotton and damages elastic and synthetic fabrics.
How do I know if it is the dryer or the washer?
Feel the clothes coming out of the washer. If they are dripping rather than damp, the washer spin is the problem -- often from overloading or an unbalanced load. If they come out normally damp and still need two dryer cycles, the problem is on the dryer side.
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