Pressure Washer Nozzles will wear out. As they do, the hole size gets larger and it takes less pressure to push the water through the larger hole. Your pump will still prouce the rated water flow but the pressure drops as the hole it is being pushed through gets larger. Or, you may simply have the wrong size nozzle.   Check here for replacement nozzles.   

Here's a hint - if you want to lower the pressure of your washer for cleaning sensitive areas, simply put on a nozzle with a larger hole. You will get less pressure and the same amount of water flow. Check here for nozzle sizing for your pressure washer.