My sulphur denitrator (sized at 0.8% of volume) dropped 40 ppm to zero in 4 weeks, after one week to cycle the denitrator. It does sound like your flow rate is in the right ballpark, so the O2-free zone must be getting disturbed in some way and not forming. Maybe upflow is the secret, as any nitrogen bubbles would rise back through the O2-free zone in a downflow design
I wouldn't worry about the volume of sulphur. If it's too small, nitrate reduction in your system will simply be slower, and as long as it is sized large enough to counter the new nitrate production, you'll reach zero. Once it's cycled, you'll be able to tell if it's large enough.
I started out with two Phosban 150 reactors (one with sulphur, one with aragonite) to test the concept on my 500 gallon system, well short of 1% volume. With the effluent nitrates at zero for this test system, I knew that a larger design was required.
Myself, I've been having trouble with low pH for the effluent, so am looking to build a recirculation chamber with aragonite on the outflow to counter the problem.
Keep persevering ...