Diagnosis
The selector switch is faulty. The switch is a multi-position rotary type. Internal contacts for the lower settings have burned out or corroded.
How to Fix It
Unplug the dryer. Remove the handle casing to expose the switch assembly. The switch is mounted to the main PCB. Spray contact cleaner (like DeoxIT D5) directly into the switch mechanism while rotating it through all settings. Work the switch 20-30 times to clear debris.
If That Didn't Work
The switch is damaged internally. You must desolder the entire switch from the PCB and replace it with an identical part. Match the pin configuration and rating.
Heads Up
This failure is often from rapidly cycling the switch under load. Always turn the dryer off at the power button before changing heat/speed settings to reduce arcing.