I use a bitzenburger for fletching and as jthain says on the collar that houses the rotating index dial that sets the vane angle there are two grub screws. One does 120 degrees between fletches and the other does 3 fletches but at a different angle setting. It may be that yours is the same. Take it out and have a look, or fletch 2 arrows, one on each setting and see if they are different.
If you find that the jig is faulty it is probably the one Hamish made on a Friday afternoon after a lunchtime whisky.
