Wrong invoice form being used when fulfilling sales orders

I am having problems with getting the correct invoice form to be used when I fulfill sales orders in a batch.

We have long used two separate invoice forms.  What I have done up to now is to have two separate roles set up. Each role has a different invoice form specified as the Preferred form. I change the role I'm using depending on which invoice form I want used.

As I say, up to now this seems to have worked (although it might have just been luck or coincidence).

But now I’ve set up a third invoice form and I can’t get the system to use this one when I’m fulfilling sales orders, even though I set up a third role and set the correct invoice form as the preferred form.