ptrick21186· Reef Geek
If you have algae growing in the tank the nitrates might be locked up in the algae as opposed to being in the water column. My tank is currently doing the same thing (4 months old). Once the algae starts dying off it will release those nitrates back into the water and your tester will be able to pick them up. You can also try increasing the feeding and skimming less aggressively.