Having a hard time getting good purple coralline algae growth

Hey guys! I am confused... I had good growth for a while but things have slowed down.. i love the purple in the tank, thats just me, i know some of you aren't that big of fans because it can take over... but I'm having a real hard time getting good growth. My water parameters are: Am:0, Nitrite:0, Nitrate:<5ppm, Testable Phos: <0.05ppm, Ca:440ppm, Mg:1350ppm, Alk:10. Is the small amount of inorganic phosphates inhibiting calcification? I know there are more phos than whats being tested for, but I also wet skim all the time. I just love the purple and would love to see better growth. Besides that, everything in my tank is clean and healthy. Thanks for the help.