Quoted from PinMonk:It's been brought up before. Ball is likely moving too fast for the opto to register. There is a fix, but I don't have a link. It just involves adjusting the opto, if I remember right.
I thought I read that someone made some spacers (or something to that effect) to raise the opto upwards where it registered better. I don't own an IM, but I've been trying to keep track of issues and fixes, as I really want to get one (possibly this year).