There are several ways of adjusting flow/pressure, potentially anyway.
Most Procon rotary pumps have a screw adjustment on them near the output of the pump. It typically is quite a big screw and pretty obvious.
The other way is via an orifice jet in the system somewhere.
The other is a suppression valve assembly which is a dampening system come overpressure relief system.
It may help to take the cover off the machine and see if the overpressure venting system is the limiting factor or not. If it is then you will see the water flowing into the tube into the drip tray. This would mean that you are getting the pressure at this point but it is being vented. If this is the case then you may need as you say a new spring or a washer to increase the pressure it will vent at.
If it isnt venting through the overpressure system then check the flow rate which should be about 100mls/10 seconds or thereabouts. I would suggest then trying to increase the flowrate at the rotary pump as described above and then re check the flow rate and pressure and see if it is all OK.