DHCP6 lease

I would start at  ;

diag debug reset

diag debug en

diag debug application dhcp6s  -1

1> review any errors 2> make sure the server is enable 3> ensure you can ping the link-local address of the dhcp6client   4> double check the client is "actually" working

follow the below link on my blog;

http://socpuppet.blogspot.com/2013/01/dhcpv6-on-fortigates.html

and

http://socpuppet.blogspot.com/2014/01/dhcpv6-fortios.html

FWIW: FortiOS has made improvement with  both dhcp6server and client on the  fgt so it would be interesting to see what you find. You might have to find out what the dhcp6client is requesting to further issue.

On a side tip, if you get into a "finger pointing" and blame the firewall I found a cool trick if you have free ports to spare,

1: cable a  FGT port to another  port on the same  fortigate in a unused vdom.

2: enable  that port in the unused vdom as a dhcp6 client  interface

3:  diagnosse the 2 ( server-client)

Doing that , rules out the host machine and let's you validate that your dhcp6server instance does works.

e.g MY home FGT100D

Port1

(dhcp6server)

  vdom root

Port2

(dhcp6client)

  vdom  mytestlab

Cross-over cable port1---->port2

Enjoy