If you know that your webservers are up, but the Balancer is showing them as down it is most likely the HTTP health check.
You can go to Groups or Backend Management and click edit on your Group/Backend.
Then go to the Protocol Checks tab and turn off HTTP checking.
You can now reload the Balancer and see if it comes right. in this state it will still check which ports are open, but not for HTTP responses.
The cause of this issue is most likely that your webserver returns a request for authorization, or a 404 not found when we send the request.
It is possible that you may need a Host Header to resolve the issue.