Here are a few things to check:
- Make sure $server_root_url in site.conf matches the desired address for connecting to the server. If the proxy is enabled then this should be set to the address of the proxy server.
- Check the apache error logs on your WeBWorK server to see if there are any enlightening error messages there.
- Check the logs on your nginx server to see if it tells you anything.
- Check your browser's error console.
If any of those places give you explicit error messages you can post and see if anyone can further troubleshoot.