If the LED lights, then there are only a few parts that could be bad. Check your solder joints around the opto, triac, and the CH 13 output connector on the terminal strip and make sure they are good.
You can try swapping the opto with one from a working channel. If that doesn't fix it then the Triac is probably bad. I have never changed one that is mounted to the heatsink but from my experience here is my suggestion. Remove the hardware for the Channel 13 triac. Bend the triac away from the heatsink and cut the leads. Heat the leads one at a time and pull them out with needle-nose pliers. Clean the solder out of the holes and put in a new triac.
Good luck