What do I need to do to get ProxyPass directives to register?
From what you are writing (or rather: not writing) the module proxy_http could be missing.
You can run apache2ctl -M to see the loaded modules.
Try adding the below. You might not need the second one if your backend is not using SSL.
SSLEngine on
SSLProxyEngine on
If you're not getting an error log, check file permissions of the log location and ensure the user which runs apache has permission. Once you get an error log, it would be easier to troubleshoot.