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
