Resource ''Create" route 404s: how do I troubleshoot?

As the screenshot shows, I have the create php file in place but clicking to create a new resource gives a 404 error. Prior to now it was working fine (even with the relations), the key change I did was to set an observer on the underlying model as well as generate a 'view' page to avoid modal. How can I troubleshoot this as there is nothing to indicate cause.