Laptop fans behave strange?

Recently, my laptop fan doesn't work as it should. I have already opened the laptop, removed dust and the fan and the cables look normal. Problem is: I watch Youtube and the CPU temperature is 70 degrees and you can hear the fan very quietly, so it runs at minimum power even though it should run stronger. Could it be that some fan control software is not running properly or something, because the fan comes on.

You can blow from the outside with a blower into the fan - please do not try in the apartment, but use the terrace / balcony.
If possible, remove the keyboard. The blowing can be done with some vacuum cleaners, but should not be done with too much pressure, but only lightly.
I use a defective hair dryer (only blower without heating), to which I have glued a tip from a silicone cartridge.
If a cloud of dust blows from the laptop and the device is still running after the abuse, you have done everything correctly. It works for me anyway.