Repair a lawn irrigation system with a defective sprinkler head by carefully digging around the head to expose the riser, unscrewing the bad sprinkler head, and installing a new one in its place. Prior to replacing the head, try unscrewing the top and cleaning it in a bucket of clean water to remove any dirt or debris that collects inside. Reassemble the head and screw it back onto the riser.
If the sprinklers are not in proper adjustment, the system misses parts of the grass. While adjustment methods vary by manufacturer, many use slotted screws on the top of the head to adjust the spray pattern and distance. Adjust these screws and check the pattern. Readjust if necessary.
If an entire section of the lawn is no longer receiving water, there is likely a zone problem. The problem might require replacing a valve or fuse. A broken or damaged supply line could also cause the issue.
A sprinkler system that seems to have low overall water pressure may have closed valves at the backflow device. For proper operation, the valve handles should be parallel with the pipes they serve. If the valves are not fully open, the sprinkler heads may not extend to their full height or remain at ground level.