@Capouch How are you making your fondant? You mentioned Grewelings book, are you using the formula from there? I've made fondant many many times, and personally have only a l experienced grainy product from agitating too soon.
Since you mentioned that you make your own, how do you agitate it? Are you certain that you are waiting until the boiled syrup is 120f or below before you start agitating?
Edit to add a few more questions:
You also mentioned the inclusion of invertase, does that liquify your centers at all?
How large are the batches that your producing?
