You may encounter a situation where health check stays in a down state even though it is not failing. Here is an example of what you may see in the sysinfo:
Domain name: test DNS status: success
Enabled Check failed DOWN
IP address: x.x.x.x Enabled OK UP
Last status: Success.
Successes (total): 16481 (last): Fri, 16 Oct 2015 09:56:40 GMT
Failures (total): 5 (last): Fri, 16 Oct 2015 01:00:33 GMT
(consecutive): 0 (external): 0
Last response time: 3 ms Average response time: 6 ms
Minimum response time: 1 ms Maximum response time: 132 ms
This all affects health checks created by forwarding hosts, such as those which are testing the TCP connectivity.