If your ammonia and nitrite are at zero 2 days after the last large water change, that is encouraging. The nitrate test may be a bit suspect since ammonia and nitrites ultimately become nitrates. Do you have so many plants in the tank that it looks like a jungle in there? Plants can remove considerable nitrogen when they are dense and growing vigorously. Otherwise, I suspect you are not doing the nitrate test quite right. The instructions that tell you to shake the bottle well very much need to be followed if not exceeded. The mixing of test chemicals for the nitrate test are very important and many people will advise you to double the times they recommend in order to get good enough mixing.