There are many things that can cause your dryer not to dry well.
1st of all you need to determine whether or not your dryer is heating. If there is absolutely no heat coming form your dryer you either have a problem with the power source or the dryer it-self. To be able to tell which one is the problem you would need a volt meter to check the power at the outlet. If there is proper voltage then you have a problem with the dryer, and would need to call an appliance repairman. If you don't have proper voltage at the outlet then you would have an electrical issue and you would need an electrician. Using an electrical meter can be dangerous if you don't know how to use it, so if you have to choose which one to call, know that approximately 75% of the time the problem is in the dryer.
Now if your dryer is heating but not drying, you probably have a venting issue. If you have an electric dryer you can disconnect the vent from the back of the dryer and dry a load. If the clothes dry then you have a venting issue, if the clothes don't dry then you would probably have a dryer issue. Now this is a great way to test the dryer, but it is best to have the venting attached to the dryer while in normal use. (Note that if you have a gas dryer, running the dryer with the vent off is not recommended).
One other thing is that if your dryer is heating but not drying, that can be a fire hazard. Dryers do start on fire! You should have it repaired ASAP!
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