1V output only when load is applied?
You are using 24VAC to feed the transistor output of an opto-coupler. That's plainly wrong and this won't work as you intend. The absolute maximum voltage rating for this device operated with a reverse polarity supply is 7 volts.

If you operated at DC 24V you are going to hit another problem and the opto might overheat. Even if you could push 12mA into the 390 ohm load the volt drop across the opto transistor would be about 19 volts and the internal power dissipation is 228mW. The device max rating for the transistor is 150mW.

The device has a current transfer ratio of about 60% - this means if you push 12mA into the diode, you can only expect 7.2mA coming out.