The 1 most common part(s) or condition(s) which cause the symptom String trimmer head won't spin are listed below starting with the most likely. Check or test each item and watch any available videos.
RepairClinic Item # 1657211 Aftermarket (Non-OEM) part
If the string trimmer head won't spin the head may have separated from the drive shaft. Disassemble the head to determine the cause.