this is a common issue.
SOL3224: HTTP health checks may fail even though the node is responding correctly
Summary there is a mismatch on the HTTP version being 0.9, 1.1 or 1.0. By default, the BIG-IP system uses HTTP 0.9 for HTTP monitor requests. When an HTTP 0.9 request is sent to an HTTP 1.1 server, the server may not respond as expected.