12 GPM should be fine for almost anything. Are you running something like a hyd brush cutter that need a higher GPM.
I would first change the hyd filter and make sure that the cover plate dose not let air into the system. I would also listen for signs of air in the system or diverter valve associated with the power steering.
I would also pressure check the hyd couplings - I believe you should get close to 3,000 psi.
If you can't get into the 2,000 range I'd look at the pressure relief valve or diverter valve.
If all of this is ok it might be time for a new pump. The pump will first start to loose it's ability to put out the state volume at the higher pressures then loose its ability to get to the higher pressure with any volume.