The brendons are set at 207 bar, the unloader will have a yellow paint mark at the top which shows if the the unloaders been turned or tampered with, if the paint mark is broken, then it has been tampered with, it may need turning, if the paint mark is in tact then don't adjust it. You should check your nozzle first to see if it's the right size, should be an 045 to create 207 bar,
How are you feeding the washer? if your feeding it from a tap then that could be the reason of low pressure as there isn't enough water, also has it's been sat it may have a cracked piston, therefore sucking in some air instead of all water.
If your sucking from a drum, make sure your hose or filter isn't blocked and restricting water.
If all is fine then look towards the revs, again there should be a yellow paint mark on the governing bolt, if this has been broken then it will have been tampered with, if it's not broke then it's set right
if you have gone through these things and it still only produces 2000psi then you may need a seal kit.
These machines are not designed to be run off a tap, hence the reason they have a suction filter.