If you're removing phosphates and have low phosphates (below 100 ppm) and not replenishing them, the BB will be inhibited. Ammonia will be the first indicator as I posted before. If you're removing kH, then your pH will decline pretty rapidly when nitrification is present. Low pH (5.5 and below) and high ammonia (above 4 ppm) will also inhibit the BB. Thus, that would make 3 separate factors that inhibit nitrification, each of which is capable of putting BB into cellular hibernation.
I don't understand why you're using R/O water, so I'm not claiming to have a better idea. But, it's use seems to be creating a condition which virtually negates BB in your system. I'd probably revisit that whole decision process rather than trying to add multiple chemicals and additives to "fix" it.
In the meantime, I'd change the feeding regimen to be small amounts daily or every other day rather than large feedings once or twice per week.
