If your screenshot shows an "Access Denied" page, this indicated that the target site has a WAF (web application firewall) blocking crawlers. The solution is to Whitelist Stillio IP addresses which requires administrative control of that site. Alternatively, your firewall may also grant us access based on a custom user-agent or cookie value. 

A workaround can be to select a different country in the URL Settings > Location list. This will use different IP addresses than the main pool of IP's from the Data Center that runs our screenshot engine. If screenshots from other Locations also get blocked, it's probably because all types of data center IP addresses aren't allowed to access the site. The best chance to bypass such firewalls is to use a proxy server/service that is not related to a data center. For this you can use our Custom Location setting and Bring Your Own Proxy.