With the power OFF, ( better yet, unplugged) gently, and I mean gently, nudge the hammer away from the crossbar with a thin screwdriver. If that solves the problem, then eventually you will need to loosen the hammer mounting screw, align the hammer away from the crossbar, then tighten it. If you are really lucky, a gentle nudge may solve the issue for a long time..
No, this does not sound like a Letoff issue, and has nothing to do with lost motion.
One more thing to check- does that key press down deeper than the others?