The part(s) or condition(s) listed below for the symptom String trimmer head won't spin are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The clutch engages one or more drive shafts which attach to the trimmer head. If the clutch assembly is worn out, it won’t be able to engage the drive shafts to rotate the trimmer head. You may be able to clean and repair the clutch assembly. However, since replacing the clutch is relatively easy and inexpensive, it may be easier to replace the clutch assembly.
The drive shaft is a long, flexible shaft which connects the clutch to the trimmer head. The drive shaft engages the trimmer head. If the drive shaft is broken, the string trimmer head won’t spin. The drive shaft is not repairable—if the drive shaft is broken, you must replace it.
If the threads on the drive shaft are stripped, the trimmer head will not stay on the shaft. If this happens, replace both the trimmer head and the drive shaft.
The throttle cable connects the throttle trigger to the carburetor. When the cable is pulled, it opens the throttle on the carburetor to accelerate the engine. If the throttle cable is broken, the engine will not accelerate, and the trimmer head won’t spin. If the throttle cable is broken, replace it.