I recommend cleaning the canister filter weekly. I also have a canister on my reef tank, and have 0 nitrates, and it has been that way for a while. Really clean out those foam pads, make sure that there is nothing left in them. Also, that white pad that goes in there, either replace it every time you clean the filter, or clean it thoroughly as well.
The problem with nitrates is that if you have 40ppm, you have to do a 50% water change just to get it down to 20ppm, and then another 50% just to get that down to 10ppm, and so on. 20% every two weeks is not going to really get you anywhere. I did 15-20 gallon PWC
every week on my 55 gallon until my nitrates were 0, along with a weekly cleaning of my canister filter. After that, try to target feed your tank, don't just dump a wad of food in there and walk away. Uneaten food quickly wrecks your water quality. Also, a good skimmer helps as well.