Monitor when set HTTP server shows down but when I set it to TCP it works

The tcp monitor is a layer 4 health monitor. It only checks that it can establish a tcp connection against the pool member.

The http monitor is a layer 7 health monitor. It does an http request (HTTP GET) against the pool member and expects an http response.

In essence the pool member is probably accepting new connections on port 80 but is not responding http requests.