Another place to check is the dhdpcd log. Does "sudo systemctl status -l dhcpcd.service" show any error messages?
You may also check whether your router does send proper RAs using the rdisc6 router discovery tool. Install it using "sudo apt-get install ndisc6". Then run "rdisc6 eth0" or "rdisc6 wlan0", depending on which interface is online and facing towards your IPv6 router.
This commands sends RS router solicitations and prints the RAs when it receives them. You should 1. receive RAs, and 2. these should contain global prefixes. In case you don't see them, something is rotten.
In case you get them, then there's probbaly a misconfiguration in network/interfaces and/or dhcpcd.conf.
From ZX81 to Raspberry Pi, but wait ... where's the 7805 gone?