You could check with ULS logs/fiddler, you may find some clues there.
Besides, Here are some articles about how to install and configure web application proxy to publish site externally. Hope they will help you.
securely-publishing-sharepoint-externally-using-web-application-proxy
securely-publishing-sharepoint-externally-using-web-applicaiton-proxy-part-2
sharepoint-and-the-web-application-proxy-role