Command of K2633 by opto couplers (PC817)

I recently bought a K2633 module for model train control purposes.
The module has been assembled and works fine (stand alone tests OK).
I tried to command the IN inputs via an opto coupler (PC 817) whose emitter is connected to the ground (the opto coupler is itself controlled by another circuit).
The opto coupler contact seems to work properly when standing alone as I measure a different value on ohmmeter when opto coupler should be ON (50 ohms) or OFF (open circuit indication).
When connected to the IN of K2633, the opto coupler does not succeed to trigger the relay contact.
I mounted the RX of 820 ohms supplied with K2633. Should I modify this resistor?
Thanks in advance for your help.
Kind regards,

Increase R5…R8 to e.g. 1K.
Should fix the problem