Hi @1000_Hills,
Welcome to the community! 
Thank you for your question and patience!
As you see in the screenshot below I can get all the info from the trackedEntityInstances. If you want to see all the fields that are available use the parameter fields=* (and then instead of the * it’s recommended to be specific).
This is the URL I tried in the 2.37 instance on play: /api/trackedEntityInstances?program=[programID]&ou=[OrganisationUnitID]&fields=*
