could you possibly include more plants,
either fake or real,preferably real,as thay help with different stuff,
on the other hand(and i do not wish to cause alarm in any way)
is there a possibility that this is a sick fish,and they have ousted
it from the group ? if the fish appears well,it could just
be that this fish is the lowest in the pecking group,and
there will not be much that you can do,apart from either
move it to another tank,or take it back to the shop.
sometimes in aggression with fish you can change
the tank decore around and this will help.
keep us informed how you get on if you can.