Just because there is a reaction, does not mean that there is actually feeding taking place. The right kinds of food are of utmost importance, and because what dictates "Right kind of food" is still almost all trial and error it is still a crap shoot.
I have been researching this topic for quite some time, and there are very few people who have had real documented success with NPS(Over one year with documented polyp growth). All those who have are constant feeders, of small quanties to keep a constant food supply. I cannot remember who said it but our goal is not to have the coral polyps extand every once in a while when we feed, but to be constantly out and feeding. Just like every other coral.