Unscrew the nozzle and look at the end (not under pressure) press the trigger and the silver piston should pull back from the rubber grommet. If it is then its likely the grommet/washer has swollen and needs replacement. It just pulls off. If there is no resistance when you press the trigger then it may not be pulling the piston back for some reason. After you have unscrewed the nozzle the top cap which houses the plunger just pulls off and you can see how it works.