There are many reasons why IPv6 might not work. One prime example is due to ICMPv6 messages being blocked (even in IPv4 blocking ICMP doesn’t really help with security, in IPv6 fully working ICMPv6 is essential for trouble-free IPv6).
If you want to get some more useful answers, then post config (at least all of /ipv6 tree) for us to review (redact sensitive data, such as WAN IPv6 addresses, but redact them in a meaningful and consistent way so that we’ll be able to see if addressing and prefix lengths are fine).