why am i showing no nitrate on test kit

i am using dry ferts and following the james tank dosing method,some of my planted are not looking to good so i thought i would test the water,cant work out why my nitrate is coming up as 0,so i thought i would double the dose of kno3 and it still com up 0,everything else is coming up fine,my tank has only bin running for 5 weeks it is a new set up,and i only have 3 tiny fish in there at the moment ,do you think i should add nitrate on its own as well as adding potassium nitrate,to get the nitrate levels up, tell i get some more fish in there , i am new to planted tanks,as you can probably work out i am not very clued up,....

240l tank ,co2 injected 4 bubba sec,3 x39 t5

dosing
kn03 1/4tps 3xweek
kh2po4 3/16 3xweek
trace1/4 3xweek
mag 8ppm
sulphate of potash 10ppm

cant work out why some of the plants look ok and 20% of them look stunted and showing sign of problems