One of your optics has died I suspect. I had the same thing happen where only one direction would be registered by the game. You can easily check it by metering the output of the Optic receivers. They should be somewhere over 4VDC. Any less and the game won't register the inputs.
They're quite easy to replace. I was unable to find a single unit optic like the original. Inside the housing are standard emitters and receivers that run on 5V. Getting the new units in the housing proved too difficult, so I just used two individual units in place to of the single original unit.
Sorry to say, I'm out of town and can't remember part numbers. Can anyone chime in with a part number?
Here are a few pictures of the replacement procedure:
Here's the original curved optic on the board:
Here's the board with the optic removed:
Here's what's inside the curved optic unit, Just standard emitters and recievers:
Here's a pic with replacement emitter/receivers installed Most standard units that run on 5V and fit the board will work: