Can't access js files through proxy server

I’ve got a perfectly functioning wordpress site running at https://parlayblog.azurewebsites.net/ (though there’s no real content yet).

I’ve set up my main website as another Azure website (https://parlayconcepts.com/), and I want it to be a proxy server so that all the wordpress content is served to the outside world from our main domain.

For example, the hello world blog post is found at this address: https://parlayconcepts.com/blog/hello-world

All the web server jiggering I’ve done appears to work properly: the browser finds/displays the content and the proxy web server has re-written all the URLs so that ‘parlayblog.azurewebsites.net’ is replaced by ‘parlayconcepts.com/blog’ (I can see that it’s all correct when viewing the html page source).

However, there are a few files that the browser can’t access, most notably the js files in the wp-includes folder. A 403 Forbidden error is returned.

Why would the original site (parlayblog.azurewebsites.net) send the article content but not the js files? Is there something special about the wp-includes folder? Do I need to loosen some permissions on it?

Any guidance or clues is much appreciated! I’m having a hard time finding anything on the internet that addresses this…