500psi is trapped pressure created by the pump the 220psi is open pressure which is created by the restriction of the jets, (if you check the flow with an open solution line it will be even lower as there are no jets to restrict flow)
The pump specification will be a set psi at a set amount of flow, if it’s a 500psi pump then you are getting the correct psi now find out the flow per minute and test it ( use an open solution line firing into a bucket for 1 minute and measure how much water collects then compare that to the pump specifications )
It can be only 1 or more of 3 things,
Restriction going into the pump
Restriction coming out
Or a faulty pump
What are you testing the working psi with? a 3 jet wand? Try a single jet hand tool see how that effects the drop in psi