One additional note, on reflection, adn please forgive me if I'm telling you something you already know.

It gets harder to focus on the grain as you stop down, to 16 and smaller, especially if the grain is fine to begin with and you're expecting something larger, as Doremus indicates. I make no pretenses to expertise in optics. However, in my experience with quality enlarging lenses, the cautions from many decades ago about focus-shifts as the aperture changes have not be relevant. I focus wide-open, or (rarely) at f/4 with my 50mm 2.8, then stop down to printing aperture.