It is NOT a phase. Tangs are hit and miss. Some will, some won't. Take it back and order a new one. Most of the time they won't, but it appears as though you didn't get lucky this time. What kind of water current do you have for him? What size is your tank. I do not have one but I know people that do. Achilles tangs NEED a TON if water movement if they are to survive.
-Adam