To troubleshoot any VPN issues, you should have knowledge of how to configure a VPN service so that you can verify whether is it related to misconfiguration or information not match.
There are few useful command can be used to troubleshoot IPSec VPN issue. To run those troubleshooting, first thing u must able enable SSH in your NSX Edge and access it via SSH.
To view full list of commands for ipsec, run command:
show service ipsec ?