Why am I unable to get 2x enlargement with the Prism lens?

Because of the stationary positioning of the glass lens within the lens barrel, combined with the convex shape of the lens, a “gap” is created when reversing the lens for either enlargement or reduction. The size of the “gap” is 1.65 to 2.24x enlargement. Therefore, 2x enlargement is not possible because it falls within this gap.