Takes long to dry clothes

Dryer has a moisture sensor that detects the clothes are dry so it shuts off earlier. If the clothes are still damp then the dryer vent may be plugged and causing the unit to run hot or not able to fully evacuate the air from the machine. Clean the vent and make sure it's completely free and try again. If it short cycles again and clothes still damp you may have a sensor controller issue.

Check for clogs in the vent air line. If dryer lint or some obstruction is slowing the air flow it will take longer to dry clothes. Also the dryer may not be getting hot enough. Check to see if the clothes feel hot or warm when you reach inside. If it feels hot enough it is likely an issue with air flow. heat and air flow are required to dry clothes.

if it is a gas dryer check your vents there is an obstruction some where. to test this you can disconnect the vent from yhe dryer and tyr drying a load.

just a quick suggestion
check your exhaust outside for proper airflow

Do you know if the dryer vent is free of obstructions?Check outside where the dryer vents outside ,and see if the air is weak or flowing freely.If it is not clogged the air should be pretty strong from the vent outside.If it has blockage it will definitly take longer to dry your clothes.

For a gas dryer:
Assuming the dryer is working properly and the control cycles are ok, look for things like a restricted venting system which inclides the exhaust, lint screen, etc. This would be the large pipe connected to the dryer exhaust that vents outside.

If it's an electric dryer, there could be issues with heating elements, thermostats, a control board, the timer, etc. Also check for any excessive lint build-up.

I'm guessing it is electric? Do the clothes get very hot? If not, one or both of your elements may be bad. If it does get hot inside after running for 15mins, then perhaps your exhaust piping is clogged up. Can you tell if the output blowing from your dryer is a weak or powerful flow?
Let me know.

there is a thermal switch it cuts out the heater element when its at temp. also so has a thermal switch that cut out the unit if it gets to hot. there will be two thermal switches or a combined one. it will have 2 to 3 wire on it and is directly connected to the the drum. one of these is the problem the thermal switch is working as its cutting out the unit. to stop a fire.

My dryer has been taking longer and longer to dry my clothes. Sometimes it takes multiple cycles before the clothes become dry. They seem to be getting warm, but are not drying as quickly as they use to.

this will be a problem with your thermostats 1 of them will have a thermal cut-out switch built onto it this thermostat needs replacing it is faulty