The phosphates staying at 0 may be a false reading, or it is below the detection level for your specific test kit. If possible get an ultra low range test kit, because you're looking to stay in the .003 to .010 range. Many kits like API will give a zero detection reading if your level is below 5 on their scale. Once you have been able to verify the actual phosphate level you can go about addressing it if it is in fact zero. First thing to consider if you get a zero reading is that the various algae types in the tank are using it faster than it being put in the tank. Once you get the nuisance algae removed you should see it start to climb a little bit. If it does not you can dose the tank with something like Aquavitro Activate for planted tanks. It's highly concentrated phosphorus and a tiny little bit goes a looooong way with the stuff. It's something like 5ml for 250g to raise it .15 ppm or something crazy like that. So depending on the size of your tank it may only take .5ml to get a register able level on your kit. A word of caution with dosing phosphate, go slow with it. Dose at most 1/2 the amount you need and repeat every few days to avoid an algae bloom.