I reread the seachem website on matrix. If it hasn't impacted your nitrates, it may be that it's consuming nitrates at an equal rate or less.Lots of great info in this thread. I just found it. I need to read it sometime when I am more wide awake so I can absorb more of it.
I have a large batch of the Seachem Matrix in a canister filter on one aquarium. It's been in there for a year or so. I can't say that it has done anything for that tank's nitrates. But I should check again.
Maybe I need to dose it with some Stability or something.
Have you checked it recently?
Edit: @ anyone -- What's the scientific name for the Nitrate consuming bacteria?