What you can do is to hasten the flow (the drips) to continuous flow and drain out this water, thereafter, test the nitrate on both the unit and the tank. Once it reaches equilibrium, adjust the flow back to drips (about 3-4 drips per sec) measure the effluent after several hours, this should be lower than the nitrate level in the tank. Monitor over the next few days.
I suspect you might have too slow a drip flow as your unit might have been matured. My experience is - once the denitrification unit is mature, the drips should be flowing, use the controller to adjust the flow to balance the denitrification according to your tanks' needs.
Cheers