If you know your webservers are up, but the Balancer is showing them as down it’s most likely the HTTP health check. You can go to Groups or Backends, and choose Edit. Then go to the Protocol Checks tab and turn off HTTP checking.
You can reload the Balancer now and see if it comes right. In this state it will still check ports are open, but not for HTTP responses.
The cause of this is most likely that your webserver returns a request for authorization, or a 404 not found when we send the request. Sometimes, you need a Host header.