Triac's tend to fail to the shorted (on) position so that is probably not the issue. I would first put the LE into test mode and see if it lights the LED for that triac then. If not, I would check solder connections on the board at that opto, LED, triac, and on the PIC. Better yet, you might want to just reflow the joints because some cold solder joints are simply impossible to see. You may also want to remove the PIC from its socket and make sure that there is no corrosion or bent pins.