Do you have additional bio filtration media in the sump - Mantis blocks, or Marine Pure, etc.Usually do wet skim and get a cup per day
Nitrate remineralisation happens in anaerobic conditions, the bacteria need to grow somewhere.
Not sure if you're using real or artificial rockwork.
The artificial rock has no where near the internal surface area of natural rock.
You could try giving it a boost with something like Equation M together with NoPox (as the carbon source).
I found MB7 or Equation M kicked the nitrate removal up a notch.