GTM - Adding windows 2008 server

I have added a windows 2008 server which is running a website, to my GTM... I have set a simple ping test as the health monitor. The GTM keeps marking the server as down.

From the CLI of the GTM I can ping the IP that of the windows server, so there is 100% a working connection.

I can run TCP dump on the GTM and can see the ICMP replies comming from the windows server.... Why would the GTM be marking it down?