* The reservoir may be empty. Check the reservoir and fill it if necessary.
* The pump may be faulty. The pump is responsible for pumping the fluid from the reservoir to the wipers. If the pump is faulty, it will need to be replaced.
* The hose may be clogged. The hose that carries the fluid from the reservoir to the wipers may be clogged. If the hose is clogged, it will need to be cleaned or replaced.
* The wiper nozzles may be clogged. The wiper nozzles are the small holes on the wiper blades that spray the fluid onto the windshield. If the nozzles are clogged, they will need to be cleaned or replaced.
If you are not comfortable performing any of these tasks yourself, you can take your Xterra to a mechanic to have the problem diagnosed and repaired.