Hi Yordan
Normally when get_related_entities is called system should ideally load the Entities from API's even if they are not present at BOL level as this is the first time you are requesting for them.
May be you can try passing the IV_MODE as BYPASSING_BUFFER and check if this would help you to get the relations.
But try debugging and investigating why it is not loading the entities in Normal Mode.
Regards
Vinod