Skimmer production problem

I have a problem with all of the skimmers i put in my tank. Iv tried a reef octopus nwb110 and now a vertex omega 150. Both skimmers never held a consistent foam head and never really pulled much. My tank is 72g with a 20g sump. Nitrates are 40ppm and phosphates are 0.04 on Hanna checker. I feed about a cube of food per day and nori every other. I just don't understand why neither skimmer will hold a foam head. I barely ever put my hand in the tank and nothing ever changes except the filter sock. The reef octopus never built a foam head at all and the vertex builds a foam head for maybe 12hrs and barely pulls anything. Maybe 10ml of almost clear water. The tank is stocked with a yellow tang, pair of clowns, pair of anthias, diamond goby, firefish, flame angel, and a cleaner shrimp. I would think atleast one of the skimmers would have pulled something with the high nutrients. Any help would be appreciated and if you need more info just ask! Thank you!