Fast lenses are difficult to focus manually on many DSLRs because the viewfinder/focusing screen often doesn’t show the very shallow depth of field you get at f/1.4. A few things can help:

• Check the viewfinder diopter adjustment first. Set it so the viewfinder display markings look perfectly sharp to your eye.
• If available, use live view with magnification for critical focus; it’s usually much more accurate than the optical viewfinder for very fast lenses.
• Consider a manual-focus-friendly focusing screen if your camera supports one. These can make focus snap into place more clearly, though they may darken the viewfinder.
• Use careful focusing technique: focus, then deliberately move slightly past focus in one direction and then the other to see where sharpness peaks.
• Be aware that at f/1.4 depth of field is extremely thin, so even slight subject or camera movement after focusing can shift the plane of focus.

Stopping down is a practical workaround, but if you need precise focus wide open, diopter adjustment, live view magnification, and possibly a better focusing screen are the main solutions.