Diagnosis
The trimmer head is spinning at a very low RPM, insufficient for cutting. This is caused by the electronic clutch system detecting a binding event or the motor being in a thermal protection state.
How to Fix It
First, ensure the trimmer head is completely clear of any wound-up debris. Second, let the tool cool down for 15-20 minutes if it has been used extensively. Third, perform a reset. Remove the battery. Press and hold the trigger for 10 seconds. Reinstall a known-good, fully-charged M18 REDLITHIUM battery.
If That Didn't Work
The problem may be a faulty trigger switch or motor controller. Try a different M18 FUEL tool with your battery to confirm it is not a battery communication issue. If the battery works in other tools, the trimmer requires service.
Heads Up
Do not attempt to disassemble the gear case to 'free' the head. The electronic clutch is not user-serviceable. Forcing it can permanently damage the motor controller.