LG main compressor not starting on forced cooling

Hi everyone.
LG are no longer providing tech support to technicians who are not LG agents in Australia.
I am hoping someone can help to give me an understanding with an issue we are having at the moment with a ducted split heat pump. Outdoor ABUW366KSA1, Indoor ABNW366KSA1.
We found the main compressor open circuit so we fitted a new compressor, capacitor, outdoor pcb, air/condensor pipe thermistor. When on cooling the main compressor does not start but the inverter compressor runs. When on heating the main compressor runs but the inverter compressor does not run. When on forced cooling the main compressor does not run, the inverter compressor runs, all indoor unit and outdoor fans run at full speed. Any ideas why the main compressor is not running on forced cooling? Also if someone could explain what is correct operation in forced cooling. eg. does the unit ignore the thermistors and just simply turn everything else on or is there more to it than that. I had to leave the site before i could collect any more info, sorry if it is lacking.
As i have mentioned in a previous post LG are not a unit i see very often. Any ideas and comments would be much appreciated.