Are you certain it's a focusing problem? Any vibration of the enlarger or baseboard will blur the print, as will negative popping caused by heat from an incandescent light source. Dust, debris and grease on the enlarging lens or in the enlarger's light path can soften the projected image. Some enlarging lenses vary slightly in their focusing between the widest aperture and the 'optimum' one (or so I've read - though these tend to be cheaper ones). Any non-parallelism of the negative carrier, lens or baseboard will also cause problems with sharpness. Cheap triplet enlarging lenses might show a variation between central and edge sharpness.
Can you lock the focus and height controls of your enlarger? Any movement or looseness there will almost certainly cause blurriness.
A trick I use is to place my cheap 'n' nasty (and probably wildly inaccurate) grain focuser atop an old piece of photo paper; it's easily forgotten that the paper had a thickness. Also, modern films tend to be almost grainless and therefore impossible to focus upon with the grain focuser.
I'm not saying any of these things *are* causing your problem, but they can affect print sharpness and should at least be considered before blaming the grain focuser. :->