Powder Blue's have a pretty notorious track record of being very difficult to keep. The keys are getting good healthy specimens that are eating in store, having an extremely well established tank (1yr plus), and keep them as stress free as possible to avoid Marine White Spot, aka Ich. Usually once they have stopped eating/are not eating it is VERY difficult to stop their demise. Does the PB have ich (white spots all over its body including fins)? I would feed the frozen Formula Two (and One), and different dried veggies until you find something it likes. But if it is not even grazing on the live rock, its odds are not good
. Sorry, hope this helps!