You don’t say what optocoupler you are using but it would be worthwhile double checking what connections it uses. Not all are the same.
If you have a voltage meter, check the voltage going in to the optocoupler and if that is OK check that it is switching.
The easiest way to check that the chip is working is by using a 3V battery, an LED, and a small resistor (100 ohm or similar). Connect the LED to the camera side (with battery and resistor)of the optocoupler then trigger as usual and see of the LED blinks.
I would also double check the connections used for the camera trigger.
Let me know how you get on.