I had the same issues with my system running bio pellets. my tank always read zero nitrates and zero phosphate, as a result, my corals are pale and not much color. I would suggest to add more fish, if possible, and feed more, also to continue removing some pellets. if you do these steps, your nitrates should slowly increase, however, you need to keep testing for phosphate (don't let phosphate to be more than 0.05 ppm)
the side effect of trying to increase nitrates is phosphates most likely will increase also, as such, unwanted algae also starting to take over the tank.
if you try these steps, for about one month and see no increase of nitrates or phosphate, you can also run protein skimmer part-time only and maybe try dosing nitrates.
GL
