I forgot to ask if you turned off "server auto detect" when you created the VIP forwarding ports. this requires that a ping test work before it will forward traffic and can take your vip offline in error.
For the debug flow basic these are some more detailed instructions. we do need to see the traffic flow.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB5536
If nothing can be found by debug flow basic try using snoop on the external interface to do a basic packet capture. This will verfiy if the packets are arriving or not.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB5411