The CPJ05B is a convertible jet pump and there are two ejector assemblies available. I bet you have the 1730, which will only build a max pressure of about 70 PSI. If you use the 1828 ejector, it will build about 85 PSI max. It will still only pump about
6-7 GPM at 60 PSI, so if you use more than that the pressure will be low.
You could upgrade to a 3/4HP with the CPJ07B. Using the 1726 ejector it can build about 100 PSI max, and will produce 9-10 GPM at 60 PSI.