Customize External monitor not working to achieve service based failover

Let me explain in detail.

We are running 3 different application on 2 IIS servers with different IPs on same port 80.

And F5 - LTM using LB to availability and LB part.

We have tried default http monitor but f5 unable to detect if we forcefully down the website from IIS server.

Second method we have tried with customize HTTP monitor, still unable to read the receive string.

Please suggest if someone have faced this issue in Past.