Opto's not triggering opponent

[The Wraith]

Four of the optos on my Street fighter table have stopped triggering the relevant feature.
They show up with a camera phone and trigger the LED on the opto board under the table when you pass your finger in front of them.
There are two "quad opto" boards under the PF and when swapped the problem does not move with the board, so I've ruled that out.
They don't appear to be on the same row or column on the drawings and so I wondered if anyone has any suggestions as to what I should be checking next, as I'm stumped.

 

[Moonraker]

I'd try checking the continunity of the wiring from the quad opto board to the backbox.

 

[The Wraith]

I'll give it a go, thanks for the reply.