If your sprinkler isn't turning properly, try cleaning the filter first:

For spray and rotary nozzles lift up the pop up riser and unscrew the nozzle then remove the filter from the bottom of the nozzle. Clean the filter and reinstall the nozzle and filter back onto the riser then turn the water on to readjust the nozzle back to its original position by twisting the lower part of the pop up riser below the nozzle.

For gear drive rotors remove the riser assembly from the canister in the ground by turning the ribbed twist cap around the top circumference of the head counter clockwise until the riser assembly will lift out of the canister. Once removed look on the bottom of the riser assembly to find the filter basket. Remove the filter basket by grabbing the center pin with a pair of needle nose pliers and pulling out. Clean and reinstall the filter basket and riser assembly back in the canister, then turn the water on and readjust the head back to its original position by twisting the LOWER part of the pop up riser BELOW the top turret clockwise to the desired location.

However, If sand or debris gets into the rotors gear drive it will wear out prematurely and at this point the entire head will need to be replaced.  Call +1-800-735-7246 for warranty replacement information.