Almost all sprinklers contain a small plastic filter. This filter is accessible by twisting the nozzle off. The plastic screen filters dirt and other debris that could clog your sprinkler nozzle. When the filter becomes caked with dirt, it slows the flow of water considerably as well as the rotation of the head. Scrubbing the filter with a nylon brush will remove the dirt and clear up performance.
Sprinkler heads are usually made out of plastic or a combination of brass and plastic. These materials can buckle, bend and break when run over by a lawnmower or car. When a sprinkler is run over, it can compress the spring inside, which can stop the rotation of the head. Smashed sprinklers also won't pop up correctly, leading to problems with even water coverage. A damaged sprinkler head will need to be replaced with a new unit.
Pop-up sprinklers, including rotating heads, use water pressure to lift the nozzle out of the ground. Small bits of sand or gravel can become lodged between the inner supply tube and the outside of the sprinkler body. When this happens, friction can build up over time and scar the outside of the supply tube. Eventually, the head will stop rotating as the friction stops the tubes from moving freely. When this happens, the entire sprinkler must be replaced.
Rotating sprinkler heads contain a number of small gears. These gears must be lubricated to move freely. In most cases, water is the lubricant. But some sprinkler heads contain a small amount of oil inside that lubricates the gears. If the oil dries out or leaks, it can cause damage to the internal gears, and the rotating motion will stop working. Since these types of sprinklers have sealed housings, you will have to replace the entire sprinkler if faulty lubrication is to blame.