When dealing with a ';Pressure'; problem always start from the source.
Then by process of eleimation, find out what is wrong.
First check the water flow/pressure from the inlet...insure the filter is not clogged.
Next check the pump....remove the hose from the pressure washer, turn on the water source and then turn on the pump. There should be a marked difference in the flow.
Keep checking...Could be a few things.
Pressure Regulator Bad
Temperature check valve bad
Internal check valves stuck/bad
Generally if it is not ';Holding'; pressure while gun is in the off postion, water is bypassing somewhere.Pressure washer will not hold pressure
You don't say what water pressure. If you are referring to the pressure coming into your house, you may have internally corroded pipes, or a pressure regulator that needs adjusting.Pressure washer will not hold pressure
Maybe your supply to the pressure washer doesn't have enough flow to keep up with the pressure pump?
Maybe you need a new regulator? Take it to a small parts mechanic or get a friend that has experience with things like this