I had the exact same problem. I took the dryer apart and vacuumed out every bit of dust I could find. I also replaced the belt, because it had a kink in it. I then put it all back together. It now dries like new.
Something else you should do: Get an analog multimeter, and set the dial to the lowest number in the OHM section. Now touch the two probes together - the needle should move all the way (or most of the way) to the right, because when you touch the probes together, you have a complete circuit, with no resistance to the flow of electricity.
Now unplug the dryer, and remove the back cover. Look for the electrical parts - each one has two (or maybe four) wires plugged into it. Checking one part at a time, unplug the two wires and put the multimeter probes on the two terminals of the electrical part. If the part is good, the needle will move to the right - probably at least halfway across the dial. Reconnect the wires to the electrical part just like they were connected. If there is a part where the needle doesn’t move, or if it moves just a little, then that part is likely bad and should be replaced. Make sure you also check the heating element - it is approx. 3” by 6”, and there will be two wires plugged into it.
If you remove a part, make sure you take lots of pictures of it while it is still hooked up and installed, so that you will be able to correctly install the new part. And take lots of descriptive notes. Also, take a picture of the label which shows the dryer’s model and serial number - you will need that information when you go to the appliance parts store.
I have had dryers which didn’t heat, or didn’t heat sufficiently, for both of the above reasons — a bad electrical part, or the dryer was clogged with dust. Either (or both) could be the cause of this problem with your dryer.
Here are some examples of analog multimeters - any will do:
The thing you want is a needle that moves when there is a complete circuit. It is very simple to watch a needle for movement; it is more complicated to look at the numbers which are displayed on a digital multimeter; this is why I prefer an analog multimeter.