Fish anorexic are some of the most frequent problem faced by us marine aquarist, however most of the time, we can't do anything. Sunken stomach, forhead and body usually means is a goner for the fish and its hard to fatten up a fish that is suffering from malnutrition.
In addition, PBRT comes from Philippine where cynide are most frequently used so many of the fishes from there do not last long or dun fair well.
There is only so much we can do
1)choose a fish that is already eating in the lfs especially if they are the finicky kind
2)try not to purchase fishes that comes from philippines unless they start changing their handling and collection method
3)start feeding finicky fishes with meaty or frozen food
4)finicky or shy fishes may need dither fishes to 'teach' them how to feed on prepared food
5)flood tank with food to encourage feeding frenzy
Anyway you can still try PBRT as they are the easier powder species of tangs ard