Problem - often they stop spraying or don't spray as well as they did. Thinking it might be a failing none return valve I spent a bit of time investigating them and found that the reason they stop, is that they get air in the trigger operated pump. The pumps or not self priming they need to be full of liquid to work. Once air is drawn in they fail.
The fix - put a finger tip over the jet outlet, to act as a one way valve and pump a few times to expel the trapped air.