Tom,
dont be so hard on yourself! I miss things on clocks and they are many times bigger, just the other day I was working on a striking problem, I had been over this clock three times already thinking I had solved the problem and yet it still stopped. I caught the problem completely by accident when I saw the fly body touching one of the plates. I cant belive I had only just spotted this. Sometimes we look too hard and sometimes think that it is another piece of the mech that isnt playing ball. Dont think for a second this will be the last time it happens to you either. All you can do to start with is look for the obvious problems, then the less obvious, it is like a sliding scale, once you hit the bottom and are lost for ideas, walk away for a bit and come back to it another time. I have lost count of the times this has happened to me, you go back for another look 3 days later and spot it and thats if an idea doesnt come to you when you were not even thinking about the watch/clock.
Another recent experience saw me shortening a suspension spring because the clock was running to0 fast – yep, I was shortening it which obviously is the exact opposite of what I should have been doing, sometimes our brains dont want to work properly, on those days, go fishing instead 
