The first thing I would check are the seam allowances where you joined the pairs of jelly roll strips together. If that was even slightly off and/or inconsistent, the center points of the diamonds will be off and they will not line up properly.
The entire quilt relies on the accuracy of that seam, but it's obviously much easier to check it BEFORE you cut the diamonds. Not sure how to advise you to fix it at this point if that's the source of your problem, sorry.
I see nothing wrong in the instructions at all, though she does 'assume' a ¼" seam allowance. Because the cuts are 45º, they should be fairly routine and easy to execute with almost any ruler.