Rotary sprinklers require a certain amount of pressure and flow to operate properly. If the rotor does not receive the pressure it needs to pop up a leaky head will result.
Here are some things to look for when a rotor doesn't pop up properly:
Problem: Broken zone piping will cause loss of pressure. Look for new trees and shrub plantings, new or repaired fences–these are a common cause for broken pipes.
Solution: Repair the broken pipe.
Problem: Excessive flow-by on the rotors will cause loss of pressure. Flow-by is the water that flushes out when the rotor first starts to pop up. Hunter rotors are designed to have zero flow-by.
Solution: Replace the riser seal and spring seat on any older rotors that may have worn seals.
Problem: The valves diaphragm could also have opening issues, debris can sometimes cause a diaphragm to not open all the way obstructing flow to that particular zone.
Solution: Maintenance or replacement of the diaphragm may be necessary depending on the extent of damage to the diaphragm. Sometimes simply flushing the valve out with the bonnet removed can solve the problem.