Hi,
I understand it's connectivity from your Local to site to CGN and you need to SNAT the traffic from your local site, using this specific source IP.
I hope you would have configured the Extended DIP as in the KB : https://kb.juniper.net/InfoCenter/index?page=content&id=KB5760 .
When it fails then whether it fails in the NAT, route , Policy match or you are not seeing the return traffic. You can take a flow basic with the specific filters and can check what's happening with the traffic.
KB for debug flow basic: https://kb.juniper.net/InfoCenter/index?page=content&id=KB12208&actp=METADATA
Thanks,
Vikas