With a petrol pressure washer, the engine speed should be set at 3400rpm. At this speed the pump should produce the specified flow (litres/min)
Assuming the pump is in good condition and you had sufficient water feed - If the flow rate is down, then either the unloader bypass valve is not functioning properly and diverting some of the water back to through the pump instead of too the lance, or the nozzles size is too small.
If you have the correct flow, then the pressure is reduced because the nozzle is too big. The pump will always produce the same flow, it is the size of the nozzle that dictates the pressure. Ideally you will have a nozzle that gives maximum pressure without reducing the flow rate. If the flow rate is reduced, you need to move up a nozzle size.