I agree with Seiser-san.
Slow down, and practice with full attention.
If you can lay your hands on a video recorder and record yourself doing the strike from a variety of angles, both quick and slow.
Wear clothes that don't hide how your body moves ie t-shirt and shorts.
The idea is that you can then play back to see if your body tenses up or moves differently when moving quick or slow.
If there is a slight dip, it may come from adjusting a grip unconsciously during a strike, with the muscles in your shoulder adjusting slightly to compensate. (plain simple bio-mechanics, which is actually frightfully complicated)
Or a shift in balance or on and on and on....
If the video idea is no good, try the same with an instructor either watching how your body moves, or one of the other students.
Another thread on here (perhaps an essay) alludes to the fact that Bokken and Jo work can act as magnifiers, so that what is subtle and small, becomes detectable when done large.
Now if you studied Iaido, which can be very precise, thats a different story, which I am not qualified to answer.