I understand that you added cycled media when starting the tank?
A month later your tank must be cycled. Is your nitrate test faulty perhaps? Do you shake the bottle well before testing? Have you tested with the same test your other tank?
It is possible that due to the larger volume of water and smaller bioload that nitrate or everything else gets diluted enough not to be detected by a test. This is especially the case if you've done water changes as well...If I cycle a tank using cycled media, it only takes days, not weeks for the tank to cycle..