unfortunately, the computer did exactly what you told it. It didn't miss the IBB, you did. I know because I have been there and done that. As I get better with the computer, I worked hard to be a better observer (is it the right batter? etc). Most times, I missed the subtle pitching change between innings, but missing a batter like that usually comes from chaos and generates more chaos. As time went on, I recruited a "back-up" who could do the computer on days when I could not be there. Before long, he sat by my side at the games and became my "spotter" helping me slow down the chaos. I can't tell you how many times I would finish the previous play and he would get me caught up with the 3 pitches I had just missed.
BTW - the Fed rule that you can "announce" an IBB simply makes things worse. I wish they would make them throw the pitches so I could keep up!
Good luck....you don't miss much, so I am sure this drove you crazy!