Have you tried shutting the recirculation down for a few weeks to see if the media will seed more completely? I don't have recirculation myself, just a single pass through of the LSM (0.6% of the tank volume ... about 3 gallons), so I'm somewhat suspect of that component of the design simply because I've not used it myself. I do, however, have recirculation on the aragonite media to get the pH back up before re-entry into the tank ...
I've shut my unit down several times in the past six months, and during start up the key to reseeding is ultra ultra slow flow, and leave it there until you have a slight smell of sulphur (with your nose right at the outflow), then crack the flow up a bit (wait a few days for the slight sulphur smell to return), and repeat ...
The amount of sulphur relative to tank volume won't affect the zero nitrate formation (after all, the bacteria don't know how big your tank is), but with less volume you definately need slower flow, which can be a major obstacle. If I was to speculate without seeing your system, I'd say the flow is likely the factor that isn't correct yet and as a result, complete seeding of the media has not occurred. As well, if the length of the denitrator isn't at least five times the diameter of the cylinder, the drip rate would need to be even slower than usual to compensate and create the anoxic zone. I use a gate valve for flow control, but those can get spendy.
That zero reading you had would indicate that a super slow flow rate would give you success. For readings, I've found that the Jungle test strips aren't affected by the sulphur. Of course, you need to extrapolate between colours rather than simply reading from a digital device
I still swear by these devices for FOWLR systems with high bio load and heavy feeding.