1. DHCPv6 uses UDP port number 546, not the old port 68 (like v4 does). Make sure that is allowed in your firewall if you are running one on the clients. Also, the server side port has changed. Make sure pfsense is set to allow outgoing port 547 for server communication.
2. Make sure your network adapter is set to listen for DHCPv6. If in Linux, you will need to enable this in your interface file. In windows, there is a check that you have to enable in the IPv6 service.
Hopefully one of these items will resolve your issue.