VIP is UP but HTTPS service not reachable

(assuming this is a web server) Can you reach the server directly? Can you reach it with curl from the CLI on the F5? What's the error you get on the browser when going through the VIP?

What health monitor do you have applied to you pool? You will get this behavior if for example you have a simple ICMP health monitor which reports the pool as up, but the web service is down.

I'd say do the following troubleshooting steps:

1. Log on to the CLI of the BIG-IP and from bash run the following command: telnet [Pool Member IP] 443 This will verify if the BIG-IP can reach the pool member and it is accepting traffic on port 443.
2. If that does not work, check the HTTPS service on the pool member or check if the traffic is blocked on a intermediary firewall when traffic is sent to the pool member.

From there you will get a good start.

Also like other people have mentioned, try and access the server directly. If this works then there is most likely a configuration issue on the BIG-IP.