Hi,
Might not be enough LED current.
If we go for 30mA.
The drop across the two LEDs = 1.6 + 1.5 = 3.1V
So series resistor = V/I = (5 - 3.1) / 0.03 = 63R,
Try 68R as your series resistor.
I = V / R = (5 - 3.1) / 68 = 0.027A or 27mA
Which is within the Max rating for Output pin Current, the LED and the opto LED.
The 330R fitted will give
(5 - 3.1) / 330 = 0.005A or 5mA
Tom.. 