There could be multiple possible causes for this.

1. check how many characters are configured for the Autocomplete function to be activated. This is setup in Preferences/General

2. check that you have a valid license key, or that your copy of DeskNow is running within the trial period

3. if you're proxying DeskNow with Apache and have mod_security installed, check if you have this directive in your httpd.conf or included files:

    SecFilterSelective REQUEST_METHOD "!^(GET|HEAD)$" chain
    SecFilterSelective HTTP_Content-Type "!(^application/x-www-form-urlencoded$|^multipart/form-data;)"

(this is part of the standard mod_security example).

If you have, add the following line between the 2 lines above:

SecFilterSelective REQUEST_URI "!DWR" chain

and reload Apache's configuration with the command

apachectl graceful

(or just restart Apache)