You will get several mixed emotions on this one. There are some that will say that BT are extremely boring, reclusive, you'll never see them, etc...
IMO and IME, they will come around when they get comfortable with their environment. My male BT was out and about immediately from the time the lights came on until they came out exploring the tank, and everything. When I added my female, she hid for two weeks, then was all over the tank just like the male was.
Try dropping a fresh clam on the shell in the tank, and walk away. Triggers love clams and with it being still, the trigger can eat it when it's comfortable instead of chasing a floating shrimp around...
Also: with you having it for 8 days now, I'm assuming it's in QT where it can eat at it's leisure without competition. If it's already in your DT, are there any other fish harassing it since it's the new fish on the block?