Manual focusing can be difficult for at least a couple of reasons:
we are not used anymore to focus manually;
the design of many modern AF lenses provides a very poor manual focusing experience.
Legacy lenses are, in this respect, often better, but you could have a stiff focusing ring. I got frustrated when trying to focus on my Star Adventurer because every time I go to touch the camera, the view gets all shaky.
This can be an indication that my tripod is not too sturdy, still, vibrations are dumpled very quickly.
Last night I remembered videomakers using digital DSLR often mount on the lens a focusing bracket of some sort, to smoothly change the focus from one plane to another.
Since I had a couple of rings used to mount guiding scopes, I tried fitting one of those on my Olympus Zuiko OM 300 f/4.5 legacy lens.
The focus is now very smooth and I can focus using only a finger, which means that while focusing the image is more stable. This should improve focus precision.
A trick to try.