Hunter rotary sprinklers operate on a “gear drive” system. When in operation, water turns a small turbine in the base of the sprinkler, which drives a series of gears that cause the head to rotate. The Hunter gear drive mechanism is sealed from dirt and debris. Age, excessive pressure, or poor water quality may eventually allow particles of contamination to enter the gear drive.
If the sprinkler is no longer rotating and the water flow coming out of the sprinkler has decreased, you can first try cleaning out the filter by removing the pop-up assembly from the main body.