If there aren't nutrients in the water column, it will have nothing to consume. It just takes time.
Think of it like this, if you had nitrates at 100 and did a 50% water change you would still have nitrates of 50. It can take a lengthy period of time with the large ro
water changes to get that number down to 0. It can get quite frustrating due to this, but patience and persistence solves the problem.