I have three of them from one of the schools that I work for. I measured the output of the photo transistors, and in all three cases, there is one wheel where it doesn't respond at all. What's strange is that there's no debris or dirt of any kind in the optical path, and it's well aligned. There are gears with tiny holes which the light shines through (or not, when the hole moves out of alignment).
If I lift out the card and put something in the gap, thatobject is read fine. And if I lift the opto just a little, so that it's reading the gear teeth instead of the holes, it works fine too. Maybe something goes out of whack or some connection gets severed when it's fully in place? I haven't figured this out yet, but I hope to. I fix electronics as a hobby, so I'll probably be able to right whatever's wrong, if I only can find the exact cause.
