The hexagon part of the injector spins on the body so you have to make sure the centre is lined up so the pipe and electrical connector is pointing straight ahead.

Remember the injector should have a copper washer on it, sometimes they get left in the head when you remove the injector.

If one is left in there a phillips (medium size) screwdriver knocked into it can fetch it out.

They can be tight so make sure you have a long breaker bar or whatever to undo it.