Hi!
We still have a depressingly low number of relays that support IPv6 (currently only ~120 of ~1900 relays). If your host supports IPv6, please enable it, especially if you run an exit! This has to be done explicitly.
https://trac.torproject.org/projects/tor/wiki/doc/IPv6RelayHowto
In short, you add:
ORPort [IPv6::address]:port IPv6Exit 1 ExitPolicy reject6 *:*
(or a more open exit policy respectively)
Thanks!