If you use the global search box to issue a geocode query, that query may return zero results in the HTML5 viewer if the geocode service is hosted on a different web host than the viewer. This can be caused by the viewer making a request to get the JSON description of the locator but sending that request through the proxy page. If the proxy page does not permit requests to the locator host, it will return a 403 forbidden message and the viewer will assume that the locator does not accept single line input queries. The 403 responses will be visible using Fiddler.
To resolve, add the locator host to the Proxy.config file for the proxy page. (typically C:\inetpub\wwwroot\Html5Viewer\proxy.config)