Hi JenniferC, It sounds like the filter cartridge is clogged by the phosfree as you suspect. Can remove the cartridge and just run without it to test flow and the gauge pressure?

I know some people run with high phosphates and it works for them providing the free chlorine level is maintained at the correct level for your stabiliser level, did the pool shop disclose all the figures for you? Personally I prefer to reduce the phosphates so the pool doesn't go green in the blink of an eye if the chlorine tank runs out but others in the US don't and run successfully.