what are you feeding your fish, how much, what kind of fish and how many?
how big is your ft and gb?
it's hasn't been uncommon for me to see nitrates low for extended periods (until harvesting)
What is your total water volume and rate of circulation? Also, from where in your flow stream do you take your test samples?
On larger systems, lower nitrate readings can be expected at the fish tank inlet than at the grow bed inlet. So, for the sake of the fish I think the sample at the fish tank inlet is more accurate. While to understand the plant and bacteria health, the samples should be taken from the bio filter or grow bed inlet.
Also, I think it is important, to take and test samples at or near the same time or times each day in consistent fashion.
I hope this helps.
Your plants could very easily eating up those nitrates and causing them to disappear. If you have a consistent source of ammonia being added (either through fish or adding it yourself), and you aren't seeing any ammonia or nitrites show up, then everything should be fine. Your nitrates should try to be kept around 5-10ppm. Increase your feeding if possible to get your numbers up. Or you can add more fish, take out some plants, or leave everything the way it is and see what happens. What are your nitrate levels testing at right now? (Ammonia and nitrite levels couldn't hurt either).