If your site has Google references built into its headers or is built on Google Compute cloud, and suddenly your StillAlive scripts are not responding to checks, it may be due to Google blocking the StillAlive network from sending checks.
This issue is outside the control of StillAlive. Google as with any service provider always retains the right to allow/disallow traffic in or out of its network. Google typically classify: this as :
"Unusual traffic from your computer network"
"Our systems have detected unusual traffic from your computer network"
According to Google
"Google Search blocks IPs, IP ranges, and in some cases, whole ISPs, when a large percent of the traffic entering our networks is abusive."
Google's blocking has been implemented due to some VPN activities that they have deemed undesirable, and as, a result sometimes traffic like that of the StillAlive checks has been grouped with this undesirable VPN traffic.
They are still in the process of adjusting their systems to separate out traffic like that of StillAlive when your scripts get caught up in these traffic blocks.
More can be read about this issue here on Google's Support forums.